Nothing is 100% safe so it seems unlikely the WHO would have said that.
Some vaccine manufacturers (Brazil's Butantan) said that [vaccines were 100% effective against COVID].
My guess is whoever wrote that thought it'd convince people to take the vaccine, subsequently save more lives and therefore the lie would be justified.
https://butantan.gov.br/noticias/vacina-do-butantan-atinge-1...
100% effective is of course dubious as well but they seem to be discussing the results of a particular study, so 100% in the sense that nobody in the study got a serious case of covid.
I still don't think you can claim a vaccine is 100% effective because it successfully prevented less than 50 middle aged doctors and nurses from dying of COVID in the short span they were monitored.