In the [Post-based metatags > Range syntax section] says that the 'favourites:' metatag searches for posts with a matching quantity of favorites.

This is false, the proper metatag for this is the 'favcount:' metatag. The example links on the page don't even work.

compare: https://e621.net/post/search?tags=favorites%3A%3C100
to: https://e621.net/post/search?tags=favcount:%3C100