Is it genuinely possible for white to checkmate black in this position?

Again:
B vs. B same color: a dead position, an immediate draw, you cannot play on, you cannot resign or lose on time.
Q vs. B, Q vs. N, R vs. B: You can play on, but only one side can lose by checkmate, lose on time, or lose by resignation.
Other cases e.g. B vs. N: you can play on, both sides can win by checkmate, on time, or by resignation.
All of this is universal, independent of player strength, time control, time on the clock, master norms, prize money.
No arbiter intervention is needed.

However, deliberately playing to lose may or may not violate 11.1, depending on player strength, time control, time on the clock, tournament standings, master norms, prize money. The arbiter may then intervene after the game and change the result.

@tpr said ^

Again:
B vs. B same color: a dead position, an immediate draw, you cannot play on, you cannot resign or lose on time.
Q vs. B, Q vs. N, R vs. B: You can play on, but only one side can lose by checkmate, lose on time, or lose by resignation.
Other cases e.g. B vs. N: you can play on, both sides can win by checkmate, on time, or by resignation.
All of this is universal, independent of player strengt...

In a queen-versus-bishop endgame, there is a theoretically forced checkmate.
In a bishop-versus-knight endgame, the theoretical outcome is a draw, though it is possible to achieve a "cooperative" checkmate.

Consequently, an arbiter might intervene either during or after the game, given that only a cooperative checkmate is possible from such a position.
The arbiter must either allow the checkmate to be reached via legal moves or declare the position a theoretical draw.
If the arbiter raises no objections while the position is being played out, they cannot do so afterward—the situation is, after all, perfectly clear.

If a checkmate is delivered in a queen-versus-bishop endgame, the arbiter will have no grounds for objection—neither during the play nor subsequently.
In this scenario, the arbiter truly plays no role.

@tpr said ^

Underpromotion to a knight works as well:

https://lichess.org/study/gZKjpXnS/u5pvSeIp

I reached the same mate position, with different moves, before seeing this reply.

If I play a game of chess against Magnus Carlsen, the theoretical outcome is probably a draw, though it is possible for Magnus Carlsen to achieve a "cooperative" checkmate.

"In a queen-versus-bishop endgame, there is a theoretically forced checkmate."

• For the queen, but not for the bishop.

"an arbiter might intervene either during or after the game"

• No, an arbiter must not intervene.

"The arbiter must either allow the checkmate to be reached via legal moves"

• Yes.

"or declare the position a theoretical draw"

• No. There is nowhere in the Laws of Chess that KB vs. KN would be a draw, it is not.

"If the arbiter raises no objections while the position is being played out, they cannot do so afterward"

• The arbiter can afterwards, depending on player strengths, time control, time on the clock, tournament standings, master norms, prize money...
  If I play Carlsen and he opens 1 f3, I reply 1...e5, and he plays 2 g4 and I deliver checkmate 2...Qh4#, then the arbiter must intervene. If I play a beginner and the same happens, then the arbiter must not intervene.

"If a checkmate is delivered in a queen-versus-bishop endgame"

• The bishop cannot deliver checkmate by any series of legal moves.

@tpr said ^

The arbiter can afterwards, depending on player strengths, time control, time on the clock, tournament standings, master norms, prize money...

In an endgame of a queen versus a bishop, the opponent cannot avoid checkmate, no matter how much they might wish to.
The arbiter cannot claim that the player lost on purpose.

In an endgame of a bishop versus a knight, it is possible to lose if one intentionally makes bad moves.

A player calls the arbiter to evaluate the position; the arbiter responds:

"I will allow you to play this position out until checkmate—which is only possible if intentionally bad moves are made."

"I am annulling the checkmate, because it was only possible if intentionally bad moves were made."

"A player calls the arbiter to evaluate the position"

• There is no rule that allows to summon the arbiter to evaluate.
  You can only summon the arbiter to claim a draw by 3-fold repetition or the 50-moves rule, or another irregularity like an illegal move, or the touch move rule, or to request a piece to promote to...

@tpr said ^

"A player calls the arbiter to evaluate the position"

• There is no rule that allows to summon the arbiter to evaluate.
  You can only summon the arbiter to claim a draw by 3-fold repetition or the 50-moves rule, or another irregularity like an illegal move, or the touch move rule, or to request a piece to promote to...

An opponent calls the arbiter to claim that the position is a draw; the arbiter issues a warning, stating that a checkmate is possible—provided that intentionally bad moves are made—and allows play to continue.
Subsequently, the arbiter annuls the checkmate, reasoning that it could only have been achieved through intentionally poor moves.

Alternatively, the arbiter may simply observe the game without intervening.

"An opponent calls the arbiter to claim that the position is a draw; the arbiter issues a warning, stating that a checkmate is possible—provided that intentionally bad moves are made—and allows play to continue."

• OK so far so good. An unjustified claim is rejected.

"the arbiter annuls the checkmate, reasoning that it could only have been achieved through intentionally poor moves."

• Possible, depending on player strengths, time control, time on the clock, tournament standings, master norms, money prizes.

"the arbiter may simply observe the game without intervening"

• Yes, that is what the arbiter should do if not summoned for some claim. However, after the game he may intervene depending on player strengths, time control, time on the clock, tournament standings, master norms, money prizes.

@tpr said ^

Possible, depending on player strengths, time control, time on the clock, tournament standings, master norms, money prizes.

At the moment he was called over by one of the participants—or, even more so, when he observed the situation himself—was he unaware of the circumstances? Did he not know the level of play or the time control?