Is it a fallacy in the server?

I was playing today when i won on time, and it said it was a draw because there was insufficient material. However, my opponent had a king and a rook against my king, which can be a checkmate. How?

he ran out of time, so he can't win. but you don't have the necessary pieces to checkmate...that's why it's a draw.

Yeah, that's a drawing resource to remember in blitz chess.  You may only have 5 seconds left, but if you can manage to capture all his pieces down to a king and a knight or just a king (insufficient material) then you can avoid a loss.

If you're about to win on time with only a few pieces left, make it more difficult (if possible) for your opponent to eliminate them.