The idea that major social ills can be solved with no governmental intervention is itself very political.
The "broad" definition is the one in most dictionaries; the narrow one you suggest seems to be the overlap of partisan tribalism with a society with a single overwhelming axis for the reason described above.
In any case, simple utterances of tribalism are already clearly off-topic on HN, whether they are centered on political ideology or not, so clearly an experiment of the type here must be targeting something broader (though apparently also narrower than the dictionary definition of poltiics.)
I think the better explanation is that a lot of tribalism is slipping through the filter, so the filter is becoming more strict for a time.
Is this a quote with a source? It's genius.
(Unsurprisingly, Arkady is my favorite character.)