This is quite surprising. I assume you mean it's offtopic for this particular discussion and not offtopic for HN in general right? Even so it seems strange to shut off a discussion like that just because it started from a tangent of a topic. What's the benefit?

Edit: there are also 2 other sibling comments that are about the same topic and left behind.

The discussion isn't shut off, it's just that users shouldn't have to scroll through a long digression about credit cards at the top of a thread about the Talos.

The context of the original comment gets lost and the [-] functionality makes skipping subthreads easy enough so I don't see the advantage.