I'm just going to explain the situation, as I see it, like a fifth grader: The ACME Specification dictates that any certificate revocation marked with "KeyCompromise" requires the CA to revoke all other certificates that share the same Private Key. It's a chain reaction.
This means that when revoking a certificate for Domain A, if one marks it compromised, all other certificates that share the key must be revoked by the CA. Because this will have an effect on other certificates, the fix to Boulder/Pebble requires proof of controlling the private key when asserting that key is compromised.
One can still revoke the certificate for other reasons, as @rmbolger noted, which do not start the chain reaction of revocation.
IMHO, the issuing ACME Account Key should be enough to trigger the chain reaction. I can think of one edge case where someone might have the issuing account key, but no longer have access to private key. This is a bit of an edge case though, and there are likely far fewer users who would benefit from supporting it, than the number of users who benefit from getting the current fix out.