RCE is only utilized to complete challenge response without CA grant our server the ACME abilities.
If you are interested you may invest some programming resources to try to implement your owned domain ACME server with a commercial CA’s API to verify my viewpoint.
Because acme tools only write challenge file into /.well-known/acme-challenge/ and It’s not possible if don’t output file into the /.well-known/pki-validation/ directory CA required. The only way we find is this.
and further topic, acme has more differences than normal CA require:
acme requires CSR submission in finalize, but traditional CA requires in new-order. *(my topic on letsencrypt forum, years ago: <<Why ACME requires domain auth first before CSR?>> https://community.letsencrypt.org/t/why-acme-requires-domain-auth-first-before-csr/98482)
acme write challenge file to /.well-known/acme-challenge/ and traditional CA write in to /.well-known/pki-validation/
acme the dns validation hostname is _acme-challange constantly, traditional CA is _<md5> or _dnsauth (ssl.com, sectigo and DigiCert, GlobalSign).