The WHO is actually just citing IHME, the University of Washington researchers who have been putting out estimates since the early days of the pandemic. They suggest Europe will hit 50% in 6 to 8 weeks. Perhaps the bigger headline should be 'IHME estimates the 50% of Americans will have had Omicron in 6 weeks".
The other things IHME is predicting about the US. I've bolded the stuff that surprises me.