If we are saying that what a character says is true because he says it, by himself.
(which I don't think is the case, btw; it depends)
We’re not necessarily saying that everything characters say is true — what we mean is that
Kaidou, with the most insane record of all,
witnessed every great pirate, fought alongside
Rocks, Whitebeard, Big Mom, and Shiki, saw the strength of
Garp and Roger, was on the same island as
Rayleigh and Gaban, fought
Oden, fought
Shanks, invaded the
most feared country by the World Government, fought
Big Mom for three days, fought
Prime Hyogoro, fought the
daimyō of Wano, (and more) — and now even fought
Imu-Saturn himself.
He became a
great pirate, one of the
Four Emperors, built a reputation that
“in a 1v1, always bet on him”, is called the
strongest living creature on land, sea, and air, became the
embodiment of strength, feared by admirals and external enemies, and is
constantly hyped with tons of statements confirming or implying that he’s
the strongest.
So when
Kaidou says he’s the strongest, it carries far more credibility than
Green Bull saying the same — because the last thing supporting Green Bull is only his
own words, while
Kaidou has countless feats and statements backing his.
Akainu says something that doesn’t even
remotely imply he’s the strongest as clearly as Kaidou does — it’s an absurd ambiguity. He said something like “I’ll sink them all into the sea,” but that doesn’t mean he’s claiming to be the strongest. “Sinking everyone” can be interpreted in many different ways about
how he intends to do it.
However, “There’s no one in the world who can defeat me” — no matter how much you try to twist it, there’s
no other valid interpretation for that.