Credibility is the quality one has when one's statements ought to be believed. OP claims the WHO lacks this quality. OP doesn't illustrate what specific statements the WHO could make that aren't trustworthy; but in the very same comment only quotes a single one from them: the fact that Covid-19 would be a pandemic.

So the only (provided) statement we should disbelieve is that Covid-19 is a pandemic. Which is precisely what OP wanted them to say. Hence the contradiction.

Obviously, this line of reasoning is tenuous, but the initial claim isn't much better, as nothing is said of what it really means for the WHO to have credibility.

I read that comment as an additional armchair semanticist making the strong claim that (exaggeration mine) the WHO should have declared the pandemic before the virus reached their own country.