Twitter does waste time in that most arguments go nowhere.
But, it still tends to lead towards a better answer as someone is likely to come up with the right answer.
If it fits the data, it will spread, even w/o expert approval, as with COVID-19
I doubt the WHO is unaware that asymptomatic carriers of the virus exist. There might be some debate as to how much they are doing the spreading of the virus, but it's a numbers game.
Right now, masks are a severely limited resource. Symptomatic patients and health care workers are without a doubt the ones who need them the most. That is to say, currently only 1% (give or take) of a population fits into one of these two categories.
Asymptomatic people is literally everyone else, 99% of the population. And the risk of them spreading the virus, all other things being equal, is less than that of symptomatic patients.
If masks were unlimited, or were infinitely reusable, then the experts would probably encourage increased use of them, along with all other forms of PPE. That's not the reality, and saving resources for folks with the greatest need seems like a reasonable recommendation for the collective whole, even if it leads to some individuals being somewhat more exposed or a minority of folks exposing more as well.