On the contrary, this is the entirety of the matter. If a client, to a public or private CA, were to ask for say, random subject attributes (O/OU/...) over ACME, they'd clearly get rejected because ACME the protocol has no way of validating authoritative ownership over these attributes.
Similarly (back to this topic), ACME the protocol (presently) has no way of validating that the server is able to perform outbound connections, and thus can any CA (within the validations performed by the ACME protocol) fully verify that extKeyUsage=clientAuth is appropriate?
Regardless, perhaps @jvanasco 's commentary that this is a better fit for the IETF discussion board is better.