I was going by this low hanging chart from the server market: https://w3techs.com/blog/entry/debian_is_now_the_most_popula... which seems credible with my memories from industry buddies at the time.

Though, as you allude, I bet a lot of CentOS installations aren't as visible, given its prevalence in things like compute clusters, dev systems, and that sort of thing.

(I have no horse in the race, my personal day-to-day use machines are mostly Arch boxes, and I regularly work on both Debian and CentOS-transitioning-to-Rocky machines.)