Yes it would have been a win for your opponent. Since the pawn can promote to a queen or a rook, and that only is enough to checkmate with just the king.
Question about automatic draws
Yes it would have been a win for your opponent. Since the pawn can promote to a queen or a rook, and that only is enough to checkmate with just the king.
Alright, thank you.

On this website, if one side only has a king left (or a bishop and/or knight), timeout is a draw if the "winning" player does it. This is also listed in the official rulebook, though it's so obscure (mainly because it almost never occurs) that even Magnus got tripped up by it once.
Lichess does it differently. As long as checkmate is theoretically possible — even if it cannot be forced (for example, king + one bishop vs. king + one opposite-colored bishop, or king vs. king + two knights) — the game continues, and timeout is still a loss.
https://www.chess.com/analysis/game/live/139357317652?tab=analysis
Had I not taken his last pawn, would've be a win for my opponent? Considering he could hypothetically eventually had enough material to win.