I believe atrettel wanted to be transparent about the nature of their reply more than protecting themselves from jeopardy. Also, this is not unsolicited as OP has asked a specific question to the community.
Are you with Proton? Maybe the documentation you need is just a lock on the sent email that says "Used key for <email> from keys.openpgp.org", and the recipient could also have a cleartext message saying "Encrypted with key for <email> from keys.openpgp.org"?
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