They don't need support at riverrun. Simply put a raid in riverrun, blocking you from support+marching it, and greyjoy has a significant card advantage over lannister on top of the +1 and ties that ensures you can't take riverrun unless he seriously threw away his cards, which I doubt since Aeron ensures greyjoy the sound, either trading victarion for the hound (trading a 3 for all the forts lannister has), or simply taking the boat entirely.

All greyjoy has to do then is stall out and wait for a mustering which ends the game. Sure, its stressful doing the complex math of (can he even come close to being able to use clegane on my knight in riverrun), but it isn't that complicated.

The thing that actually stops Greyjoy is lannister outbidding them for the sword so that lannister can pretend like they have decent cards, but good luck with that with all the power tokens you'll be able to farm, lol. Any Greyjoy who is willing to throw away the sword to get star orders deserves to lose. Why would you bother with more units when a knight with sword and any of greyjoys cards bar asha is enough to win any battle with force <6? Any mustering will get you more than enough units, since almost all the places you own are strongholds...!

It might not even be that Greyjoy is OP to the point they are insurmountable and Lannister's defeat is inevitable. There's ways to stop Greyjoy from killing you but not ways to win the game without Greyjoy screwing up.