> Unlike what most people seem to think, ACME isn't something invented just for Let's Encrypt. Let's Encrypt was certainly the first high-profile CA to implement the protocol, but various CAs (free and paid) have their own ACME servers and have had them for ages now. It's a generic protocol for certificate authorities to securely do domain validation and certificate provisioning that Let's Encrypt implemented first.

This depends on whether you're speaking as a matter of history. ACME was originally invented and implemented by the Let's Encrypt team, but in the hope that it could become an open standard that would be used by other CAs. That hope was eventually borne out.