"If a Diplomat is present on a planet about to be invaded by enemy Ground Forces during the Planetary Invasion step, the Diplomat may delay the invasion by one round. Simply return the enemy Ground Force units to their Carrier(s)."
If the invading force has (only or partly) MUs... is the invasion not delayed?
...seems reasonable, though I have to say that "common" sense is at times "uncommon" when it comes to the list of convoluted exceptions in TI3/FFG. Sometimes MU's are treated as GF's, sometimes they are not, that's about as consistent as it gets, same with fighters, sometimes they are "ships," sometimes they are not; sometimes certain pre-combat abilities are part of "Combat," sometimes they are not.