Hey InboxReads creator here. I've been facing the same problem with other newsletters submitted to the site so I think you're right that spammers are scraping directories.

I've added a bunch of mitigations but found that they're faking the referrer when subscribing to newsletters and not even going through the site anymore.

Captchas do solve it though!

But I'm just as baffled at their endgame as you are.