It doesn't take much rough ground to make a chain slap against a chainguard that is fairly close to the chain.

My old 7-speed derailleur cruiser would have plenty of chain to guard slap over rough ground.

My current single speed can have chain slap against the chain guard while going over rough ground as well. I like to cut through grass/dirt fields quite a bit and there is always a bit of slap. Not as much chain slap as the 7-speed though.