I think the tone of the post was a bit combative (which is unnecessary), however, the posts clearly fit the guidelines of HN and were on a whole relatively interesting. There's nothing wrong with their posting habits.
They quoted the number of posts they made on their blog but they didn't address whether: 1) they submitted more of their own blog posts to HN or 2) changed the content of their blog posts to appeal to HN readers more. I think they probably did (which is fine with me).
I agree, they will inevitably get posted. And people should be able to post whatever they like to their own sites, without being accused of manipulating other sites.
I think your first paragraph nails it: the posts fit the guidelines and were on the whole relatively interesting and a sufficient amount of people thought so for the posts to reach the front page.
Skimming their archives, they were't posting frequently towards the back end of 2011 but if they were pushing to finish Basecamp it's understandable.
In relation to the content of the articles, I think they were more attractive to the HN community - behind the scenes look at development; interesting technical articles on development.
They quoted the number of posts they made on their blog but they didn't address whether: 1) they submitted more of their own blog posts to HN or 2) changed the content of their blog posts to appeal to HN readers more. I think they probably did (which is fine with me).