He does have a point, in that he’s not obliged to accommodate users

He might not be obligated to “accommodate” users, but I’d argue he is obligated to inform users.

If he’d made the blocklist public and an acknowledged ‘feature’ of his frontend from the beginning, then sure. Everything would be fine. Some users might want to use it without the blocklist, but he’s not obliged to provide that for them.

Making the blocklist secret, though, is deceptive, manipulative, and underhanded. And it goes against the entire ethos of both open source and the fediverse.