It's not brilliant but it's "brilliant" under Chess.com's new definition four years ago. https://support.chess.com/en/articles/8572705-how-are-moves-classified-what-is-a-blunder-or-brilliant-etc
Chess.com told the computer to call a move "brilliant" if it was a good piece sacrifice that wouldn't put you in a losing position. That's all the b3 move is. It sacrifices your rook on a1 for the bishop on g7.
The computer evaluation shows that it was not the best move but it was not a bad move.
https://www.chess.com/de/analysis/game/live/144063103288/review?move=24&tab=review&classification=brilliant&autorun=true