>>9952281vaguely: i finished preparing all the umisea shit, so now i can probably just sit back and enjoy
the last part, フラグじゃないよね, "isn't this a flag?"
is a sort of japanese slang/meme where something you say will definitely cause some sort of consequence.
for example, if a character in an anime says something like "i'll definitely come back to you" before going off to war, nips will think "that raised a death flag", meaning that person is going to die for sure.
in this context, her saying that she's finally finished and then mentioning that it raised a flag indicates that she is going to have more preparations to do in the near future, whether that be umisea or some other group.
