[PhantomGremlin]

rate-limiting is manually applied by the moderators

Given:

1) HN was started by / is moderated by "computer nerds" (IMO you're hardcore if you write it in lisp)

2) 'dang' is the only acknowledged public moderator (some small? number of additional secret moderators also exist)

It follows that:

It's difficult and doesn’t scale to "manually" apply rate limiting. Most / all such action is highly automated.

At least that's my wild, possibly way-off-base guess.