Author here. The risk with spoofing is that someone might register an email address that they can't actually send mail from.
You're right that SPF and DKIM are not universal (besides not being strictly for user authentication); this scheme would require a domain to have both in order to be secure and would require some kind of policy attestation around email local parts, which would exclude some email providers and some users. That's why it's "a random idea I had on the subway," and not something I'm doing in production :-)
You're right that SPF and DKIM are not universal (besides not being strictly for user authentication); this scheme would require a domain to have both in order to be secure and would require some kind of policy attestation around email local parts, which would exclude some email providers and some users. That's why it's "a random idea I had on the subway," and not something I'm doing in production :-)