Best available is not as effective as working from the worst case scenario assumption. Either the desease is not that bad or they didn't care. Wearing a mask and staying home is safer than doing none of that and continuing life as normal.
The WHO gives the current state of knowledge, including uncertainties. So in early January, when they didn't know if there was sustained human-to-human transmission of the virus, they said they didn't know, but advised doctors to take precautions to prevent transmission.