Is the "strategic error" losing both sunset sea and ironman bay in the same turn ignoring a large ship buildup by both tyrell and lannister?
Seems hard to pull off (since they have to break sunset sea with 2 cross supports going on), unless the supply line was left deliberately weak. Still seems weird to me regardless. The turn limit usually is what keeps greyjoy from winning in the games i've played (mainly due to supply values not getting updated in the same turn that troops are mustered), since ties in castles get determined by supply then power stored up. Actually, something like that just seems like it would be telegraphed way too much since Tyrell starts really out of position for a move like that. While both parties would be sacrificing a fair bit of land power potential to make that work.
It just seems off. Even in a turn where march +1 and support orders can be played, to jump him suddenly with 2 sets of 3 boats with tyrell taking the lead and trying to knight of flowers the move forcing the issue to be addressed on pike the next turn, Greyjoy either has to not have access to the valyrian steel blade and crow's eye, or a defense +2 order and the valyrian steel blade, or crow's eye and a defense +2 order etc. (assuming boats steup ideally in a 2-1-2 formation for support + bridge). Failing that allows the potential for Greyjoy to counter in the following turn and push the boats out again preventing the siege (turn order dependent).
I completely understand them getting contained or pushed off the mainland, but losing pike just seems nearly a fantasy much in the same way Baratheon can troll folks trying to attack him from the sea with him just blowing off the first "real" attack with Salladhor and adjusting his strategy afterwards in the next turn. Avoiding Salladhor trolling is a many turns in advance war of attrition plan which in any other game would have meant an auto win by the player pulling off that gambit.