That would expose the bot's operator to some massive liability, and I'd guess the mods would boot it with the first DMCA claim coming in.

Surely the HN moderators are not legally obligated to chase rogue bots on multiple usernames for unrelated third parties. It wouldn't be "spammy" so they'd have no need to react to it for their own moderation duties. The DMCA claims seem thin in any case especially considering SCOTUS's reaffirmation of Section 230. I don't think iterating through hypothetical legal arguments is very useful without the attendant analysis.