It is a report dated 19th February! Also note that this estimate is based exclusively on the Chinese data(!) and that it practically confirms that the Chinese data were usable for the estimates even as there were doubts spread by the media and politicians. Specifically in the same report it is written (1):

"Using an estimated number of total infections, the Infection Fatality Ratio can be calculated. This represents the fraction of all infections (both diagnosed and undiagnosed) that result in death. Based on these available analyses, current IFR estimates range from 0.3% to 1%."

And then:

"Without population-based serologic studies, it is not yet possible to know what proportion of the population has been infected with COVID-19."

So even without the population-based antibody (that is what "serologic" means) data, and only using the Chinese data "as of 11 February" available to the WHO the estimate was quite good. The range depends of course also on the age structure of the population -- the countries with much more young people would have closer to the lower number, the others closer to the higher.

1) Edit: the parent post quoted the same segment later too