"Pumping" in this context is a very thinly veiled pejorative. The original question is using it in the sense of a "pump and dump" [1]. They're implying that the amount of Rust content on HN is in some way artificial: it's the strike force[2] posting a bunch of articles, or its some startup throwing "made in Rust" in their title for attention, etc.
As you might guess, it's hard to distinguish between a "pump" and general interest in something. And, of course, each probably does feedback into other.
For Javascript it's the opposite; only one is about something written in Javascript. The rest are about language or its usage.
Not sure if one can say that that's an indication of pumping. Seems more an indication of usage.