Yes, seems you are right. Not sure where I got the impression.

Unrelated, when I researched this I saw that acme.sh, zerossl, and a bunch of other acme clients are owned by the same entity, "Stack Holdings"/"apilayer.com". According to this, zerossl also has some limitations over letsencrypt in account requirements and limits on free certificates.

By using ZeroSSL's ACME feature, you will be able to generate an unlimited amount of 90-day SSL certificates at no charge, also supporting multi-domain certificates and wildcards. Each certificate you create will be stored in your ZeroSSL account.

It is suspicious that they impose so many restrictions then waive most on the acme api, where they presumably could not compete otherwise. On their gui they allow only 3 certificates and don't allow multi-domain at all. Then even in the acme client they somehow push an account into the process.

[...] for using our ACME service you have to create and use EAB (External Account Binding) credentials within your ZeroSSL dashboard.

EAB credentials are limited to a maximum per user/per day. [This might be for creating them, not uses per credential, unsure how to interpret this.]

This all does make me slightly worry this block around apilayer.com will fall before letsencrypt does.

Other than letsencrypt and zerossl, this page also lists no other full equivalents for what letsencrypt does.