Well having just looked at that paper, which is
here, it seems to be a review which was only published on 01 June 2020.
If this evidence was not available previously, how can the WHO make the recommendation that facial coverings should be compulsory in public? If the WHO has changed its position based on this study (if we assume its a good study and for now I will assume this is to be the case), then in my view that is how science should be.
If there is a large study that was carried out that showed similar findings many weeks ago and the WHO ignored it, then your point may have better traction. There may well be for all I know but I am not a biologist or epidemiologist so I wouldn't know.
There is also a caveat in that study:
Wearing masks outdoors will probably have less benefit than wearing it indoors.