I think it's an acceptable response. Not only there's no SLA, but people are free to not provide a service to misbehaving user agents. It's like rejecting connections from Tor.
If anything, a 429 is a nice heads up. It could have been worse; she could have redirected those requests to a separate URL with an... unpleasant content, like a certain domain that redirects to I-don't-know-what whenever they detect the Referer header is from HN.