Certainly, in the balance of probabilities the WHO's guidance is more likely to be correct than incorrect. It's certainly more likely to be correct than a layperson's.

But it's not SO overwhelmingly likely to be correct that all of the big 3 social networks (Twitter, Facebook, YouTube) need to have a policy of aggressively removing anyone who contradicts them.

Even though I agree the WHO is probably right about most things, it's far below the level of certainty you'd need to justify these overreactions. It doesn't even have the intended effect of preventing irresponsible behavior by gullible people; the COVID19 hoaxers are getting way more ammunition out of the media's censorious behaviour than they would out of Trump Jr's bad medical advice.

Mostly is it on COVID and Trump posts. Hide or flagging is the best response to the entire thread. These topics do not come out well here.

The big tech companies in question (Twitter, Facebook, YouTube, etc) are using the WHO's guidance as the barometer for what does and does not constitute "misleading information" when it comes to deciding what to block. So, OP's point is not misguided. It's not like Twitter's own medical staff are the ones deciding whether hydroxychloroquine is or is not effective.