The black pawn on b2 is eating a lot of possible moves for the other pieces…

It has only one legal move, take the Knight on c1. If that pawn wasn't there it would free that square for 4 white queens and a Knight. But of course the black king would already be in checkmate so these moves wouldn't really be available.

Tempting to put that e5 Queen elsewhere so that it doesn't immediately checkmate and leave the b2 square available for others.

edit: I imagine that pawn also needs to survive that far in order to avoid a stalemate.

I was also confused about "black pieces are useless" as 2 white queens looking at each other can be replaced with 1 white and 1 black to add moves about eating each other - but then I realized it's simply "only 1 side can move"

The black b2 pawn has no moves in the position with white to move. If it were black's move, it would still have no moves since it is pinned by the white queen on e5. If it were not pinned it would have 4 moves, as it can also underpromote.

It's white to move. If black is in check with white to move, that makes the position illegal, and unreachable -- there's no possible legal move by black that led to this position where he is in check.

Replacing one of the black pawns by a white knight would add some moves, but there is no budget for that -- both knights are already on the board, and all pawns were promoted to queens. (And replacing both pawns would again make it impossible for black to have made the previous move)