I think the previous post is talking about a search that will find the sibling domain names that have obtained certificates with the same account ID. That is a strong indication that those domains are in the same certificate renewal pipeline, most likely on the same physical/virtual server.

Run ACME inside a Docker container, one instance (and credentials) for each domain name. Doesn't consume much resources. The real problem is IP addresses anyway, CT logs "thankfully" feed information to every bad actor in real time, which makes data mining trivially easy.