> The internet was made for humans exclusively, designed to keep machines out by default.

This feels like a wrong assumption. Internet was not intended for humans explicitly. If anything browsers were the explicit medium made to allow the humans to interact with internet in safe manner.

> Every signup flow assumes a browser, a person reading a page, and clicking a confirmation link. Unless agents can't do that, they can't be first class users of the internet.

This again feels like a misconception. The systems just work with an identity verified by credentials, it doesn't matter if its a program or program prompted by a human that uses it

As somebody who spent a lot of time trying to get an agent to register an email address, their assumptions were correct. It is a PITA.