Why would they have to upload it to protonmail? OP is not a protonmail user, is he? It would be ridiculous if encrypted communication would require explicitly u

Why would they have to upload it to protonmail? OP is not a protonmail user, is he? It would be ridiculous if encrypted communication would require explicitly uploading your key to each system from which someone might conceivably try to message you. You opt in by having an email address and by having a published key associated with that email address.