The normal understanding is that people develop antibodies to viruses usually by fighting them off- either through a weakened form of them via vaccination or just by acquiring and fighting through the virus normally. Herd immunity pre-Covid was understood to mean when enough of a population had these antibodies to effectively prevent the virus from being able to spread.

The recent change on the WHO site was to remove the thought that people could get antibodies on their own, and any herd immunity must be acquired via vaccination.