Anchor never see sees your private keys for certificates.
We hold an ACME account key on your behalf with the CA, but we cannot use it impersonate your domain or decrypt traffic.
We have a more technical overview of how this works in our docs: https://anchor.dev/docs/public-certs/acme-relay
That makes no sense whatsoever. If you have an ACME account key for my domain, of course you can use it to impersonate my domain. You just need to create another certificate. (Which I could detect, but if I know how to do that, I'm probably not going to need your service anyway.)
Whether or not something like this makes sense to you is probably a question of your personal threat model.
https://crt.sh for point in time checks, https://sslboard.com for comprehensive oversight (disclosure: I'm the founder)
With this we could issue or revoke a new certificate, but we couldn't impersonate them because we don't control the rest of their DNS.
If that were true, nobody would need signed certificates in the first place.