Not sure why UD thinks 13. bxc3 is the obvious move.
The reasons I think Nxc3 is better are:
a2 pawn has a pal beside him for protection still
White just forced his opponent to waste a move withdrawing his queen
His knight offers more control of the board from c3 than e4 potentially? He already has d6, f6, g5 controlled by his pawns and cannot safely move his knight there anyway.
Black's pawn on c5 is the only reason to keep his knight on e4, and that is easily defended by the b pawn if needed.
Either that or he is correct, and we are both missing something