I'm pretty sure he refers to hashcash. And while you are right, PoW is not widely used in email, I think the quote just wants to say that it's used the same way

I'm pretty sure he refers to hashcash. And while you are right, PoW is not widely used in email, I think the quote just wants to say that it's used the same way as it's used with email if/when used with email. (So the claim does not intend to be that PoW is used with email generally.)