15. Qd4 was brilliant?

It does satisfy the piece sacrifice condition, but how can going for mate in one move brilliant?

Because a decent piece sacrifice is the main condition since they changed their algorithm in 2021.

That's weird. I think they should add some calculation factor. It's not a good way to market the diamond membership then. If I upgrade, I would want brilliant to be more like something along the lines of 4 or more moves where mate would be the 4th and you had to think of 3 moves ahead before. There also a lot of games I played where it was positionally confusing, but an oddball king or pawn move prevented something. For example, I played this game yesterday where I had to move my king multiple times and give up castling where at times (maybe not all) the king only had one square to go to. Otherwise, the opponent's knights in the center would fork capturing my queen. That's the kind of brilliancy chess.com should be evaluating.