But judging from things like the number of stars (the true and only measure of quality, popularity etc.) at github
Oh, of course, case closed :-)
GNU prolog seems to be more or less a one-man-show
Well, he is more prolific, but SWI-prolog seems to mostly be Jan, but then Lexilla shows as almost all Neil because of the patch not PR process the project has used in the past, so it may be unfair to both prologs.