In universes with a wide range of characters on different tiers of power (such as Marvel and DC), this has to be evaluated on an individual character basis, instead of on the verse as a whole.
For example, if someone in One Piece claimed that they could destroy an island, it's reasonable to consider it a possibility, since other people in the verse (ex.
Whitebeard) have done comparable things (of course, it still has to be evaluated based on the feats and power-scaling of the actual character).
On the other hand, if someone said
Spider-Man could destroy a star, it's unreasonable to think this is true despite the fact that Herald - level characters and above in Marvel have done so, because Spider-Man is established as being on a lower tier than such characters.