Apparently, neither: https://support.chess.com/article/128-what-does-insufficient-mating-material-mean
insufficient material - FIDE or USCF

@arbivara's link is a good one. It's important to understand that these are each different rule systems:
- FIDE
- USChess
- Chess.com
Our rules are adapted to the unique circumstances of online play, particularly blitz and bullet, and are focused on providing the best possible experience for our members, most of whom aren't familiar with USChess or FIDE rules.
Does chesscom uses USCF or FIDE rules when it comes to insufficient material?