You're absolutely right! Edited the gp from "strictly dominate" to "are probably even with". My memory of the piece was playing tricks on me. That was a serious error, thanks for catching it.
My best guess based on a rereading of the footnotes is that the performance ceiling for chess is probably low enough that it has been ~reached by both centaur teams and pure engines. So the two would have been operating neck-and-neck as of ~2017, with win rates largely determined by random-ish factors like human mis-clicks (also mentioned in the article).
I feel like I remember even in 2017 correspondence chess players in centaur-allowed matches had centaurs beating people who expected to just be able to set up a cluster and run an engine and copy its moves. And gwern's article seems to me pretty strong evidence that the best centaur players still held an edge in all the tournaments he listed.
I am very curious to hear how that's still possible (or to learn that in fact it is now impossible), especially in the post Leela/Alpha world.
My best guess based on a rereading of the footnotes is that the performance ceiling for chess is probably low enough that it has been ~reached by both centaur teams and pure engines. So the two would have been operating neck-and-neck as of ~2017, with win rates largely determined by random-ish factors like human mis-clicks (also mentioned in the article).