> This protocol solves a pretty contrived problem ("By sending the email verification code, the inbox provider knows the user is using that service!")

I agree with a lot of what you are saying, but I think the main motivation is actually trying to reduce friction for the user to verify their email, which is good for the user, because it makes registration easier, and good for the company, because less users bounce at the email registration step.

But yeah, this is quite complicated, and there isn't a lot of motivation for email providers to implement it.