From what I gathered from their guidance the basically say masks are not effective enough to protect you & people do not know how to use them.
Well, the problem is that how well the mask protects you is at this point int time actually a secondary concern. The biggest issue is protecting others from you when you are infected and do not know that yet, which is a very big problem for covid-19 due to many asymptomatic carriers.
Even a piece of cloth covering your mount and nose can reduce the amount of infected droplets you generate be sneezing and breathing significantly if not totally stop transmission barring really close physical contact, so there is really no good reason for everyone not to wear a mask.
That WHO is totally neglecting this aspect of the mask protecting others is almost criminal negligence.