I think it definitely can still happen, as there are no definitive ways totally prevent it. But imho the chances are less likely because the nature of fediverse where the communities are scattered (which some people feel is a weak point of fediverse) act as cushion to minimize the impact. for example i think it's highly unlikely a mod in programming.dev community is also a a mod in sh.itjust.works.
But let's just say what if that guy managed to apply as mod in both place? it's also quite unlikely if sh.itjust.works admin agree with the mod's abusive action that programming.dev's admin would think the same way.

yes, it's an uphill problem. but i would argue on lemmy or fediverse in general the slope is not as steep. For example if you're banned in !games@lemmy.world you can just move on to !games@sh.itjust.works. banned there too? go to !gaming@lemmy.ml. places like piefed included those 3 communities into the same topic so wherever you posted people that goes to piefed gaming topic would still see your post. But then again if you find yourself eventually getting banned no matter where you bounce to, ESPECIALLY in fediverse then you have to look into the most common denominator...