Perhaps the time limit would not be set in stone but would be random. That way, it's not as simple as "always turtle." If you know the match will never go past 20 minutes, it's pretty obvious when (as the lower-rated player) you should shift everything into turtling and you don't care if you lose 80% of the map by 19:30
On the other hand, if there's a 20% chance the game ends at minute 20, and a 20% chance it ends at minute 24, and a 20% chance it ends at 28, and a 20% chance it ends at 32, and it definitely ends by minute 36, and the players don't know which it is (20/24/28/32/36) until the game ends, the defensive player has to work harder to balance being defensive vs playing the long game. While still giving them a "free" win if they can hold out.