I think this is largely a matter of who chooses the update the college’s Wikipedia page. For example, the list mentions that Pitzer only has 41 “notable alumni”, while Weslayan has 912 “notable alumni.” The 41 at Pitzer are the names listed on the Pitzer notable alumni Wikipedia page at https://en.wikipedia.org/wiki/Pitzer_College#Notable_alumni . There are certainly far more than 41 Pitzer grads with Wikipedia pages, but only 41 were linked to the Pitzer college page at the time of the count.
However, whoever updated Welsayan’s page at https://en.wikipedia.org/wiki/List_of_Wesleyan_University_people#Alumni provides a far more extensive list of Alumni including categories for alumni who have won awards, alumni who are professors or scholars, alumni who have founded businesses, alumni who are judges, alumni who are authors, alumni who are activists, alumni who are athletes, etc. I don’t care to count them all, but I trust that there are close to 912, as listed in the ranking page. I’m sure Pitzer also has grads in all of these categories but most are not linked on the colleges Wikipedia page. If someone reading this thread updates Pitzer’s page to link to more alumni, then Pitzer could also have hundreds of links for Pitzer too… perhaps even exceeding Weslayan’s count.
They appear to just be counting the number of alumni links from the college’s Wikipedia page, which makes it straightforward to do for non-LACs, as well. For example Harvard’s alumni page is at https://en.wikipedia.org/wiki/List_of_Harvard_University_people. It is ~60 screen lengths on my laptop. Stanford’s alumni page is at https://en.wikipedia.org/wiki/List_of_Stanford_University_people . It is ~80 screen lengths on my laptop. So if non-LACs were includied on the ranking list, I’d expect they’d list Stanford’s “notable alumni” count to be higher number than Harvard’s. This is obviously a silly way to rank colleges.