It's probably hard to tell, but I would assume HN is the least fertile spamming ground for miles around - we all seem to have fairly well tuned netizen antennae
That said what if you turned up the requirements - for example a uuid in a TXT entry on a domain. I would be surprised if 75% of HN do not already own one and 90+ be capable of getting and managing one?
Presumably a throw away domain is more costly than a throw away email?
For upvotes, submissions, and replies, the login page also has as a signup form. However, there seems to be a secondary login page (linked to at the top right) that has no such form as is shown in that screenshot in the comment you replied to.