a) I agree email is forked up.
b) I think a stamps-like solution could possibly work, but it's not the email part that makes it hard, it's the ecosystem around email that makes it hard. For example, once you send an attenuated capability to someone else, you've lost it, which means your system needs to be aware that what you're sending is a capability token and not an identity token.
If what I've said doesn't make sense- I'll explain a bit later on in this post.
c) I think it's nice to think creatively about message systems, including email, XMPP, ActivityPub
Now I'll say where I see the problems here being:
1. Overlaying on top of email
The problem with a protocol such as email is once it's in place, it's in place. Stamps (which is the idea the author references) could maybe be overlayed on ActivityPub- but I worry the time for that ship is either closing or it's sailed. I have many more thoughts on this point but it would be pages of thinking, so I'll leave it at that.
2. Cryptography not required
The author wants public keys, emails signed, etc. You don't need any of that for this to work.
What I proposed for AP (and still do) is that we can have a "public" address and then private, revocable addresses. You can then combine that with a bearer token exchange system. It's actually very simple, though I'm oversimplifying it too much here.
3. The author says I got it from Christine Webber, and "who knows where she got it from"- She got it from Mark Miller's Ph.D thesis, as well as the work done by Chip Morningstar. The theory behind all of this is called Object-capability Theory (https://en.wikipedia.org/wiki/Object-capability_model)
The email addresses are what the OCAP folks would call a capability token- the ability to send an email is a capability, and the special email I give someone is a capability token, a bearer token in this case (though there are other patterns in the OCAP world).
I can then make other attenuated capabilities, ie proxy capabilities that may act slightly different or serve a specific purpose- like a share link that only works for a week, or a share link that's upload only, etc.
Why is all this important? Because understanding the theory will help you understand the possibilities, and also the possible flaws.
Am happy to discuss further.