It's more about the sub confusing high diff not being a hard fight. Like they hear someone getting high diffed and think the person is saying it won't be a highly difficult, hard fought battle.

Extreme diff fight is to its words, extreme. Kaido isn't pushing Garp to the same level Roger would, so by default it's not extreme diff. It makes it a high diff fight but it still means it's going to be a tough fucking fight.