If you own both https://www.happy-rainbow-nursery.example/ and https://hardcore.bdsm-videos.example/ you probably go to some lengths to avoid visitors realising the connection. Nothing you're doing is illegal or even unethical - but it's obviously going to cause uncomfortable conversations so why not avoid that altogether. Let's Encrypt aren't doing you a favour if tomorrow a mom at nursery says now she knows why you sound so much like Masked Mistress Martha...
I'm guessing customer IDs are associated with e-mail addresses? This seems like a good case of using different e-mails for ever cert. There are open source tools like anonaddy.com you can host yourself or buy from them (they have a decent free tier).
I feel like this list seriously needs to be pulled. There is some serious lack of oversight here.
They are (on Let's Encrypt's end), if an email address was provided.
It's a 1:n relation, the same email may be used for any number of ACME accounts. Roughly speaking, for most clients, the ACME account maps to a specific ACME client on a specific host. If you run three servers with separate ACME clients, you're probably using three ACME accounts (even if you're using the same email and issuing certificates for the same domain).
Large or custom implementations may reuse the same ACME account across many servers and domains. (Issuance would typically be centralized and operated as a separate system in these scenarios.)