IDK
A weird bug in a daily game

Wait, it's because whover set that game up did it incorrectly. The FEN shows castling is possible, so that's likely why it reads the final position is checkmate.

How is this mate ?
But in the game , it shows that Black won by checkmate
https://www.chess.com/game/daily/366956199
he be using hacks (maybe)

How is this mate ?
But in the game , it shows that Black won by checkmate
https://www.chess.com/game/daily/366956199
he be using hacks (maybe)
It's not a hack, but apparently a bug with checkmate detection when the custom position is incorrectly created.
How is this mate ?
But in the game , it shows that Black won by checkmate
https://www.chess.com/game/daily/366956199