By the way, I continue to think that there are three of them!
I also think this development really shows ACME's credibility as an interoperable standard.
(In very early mock-ups of Certbot, we had the idea that users would be prompted to choose from a list of compatible CAs when they requested a certificate... while I'm not certain that that would give users a clearly better experience, it's cool to consider that it's now theoretically possible.)