Why is it the Tinkers, in particular, are Scrupulous? It's not particularly the first personality trait one thinks of in a hegemonising swarm.

I would guess it is because they are meticulous and precise, one of the meanings of scrupulous.

If we really drill into the meaning of scrupulous, one possible explanation is that the Tinkers are indeed principled, it's just that their "principles" are not what we would consider virtuous and lead them to being a hegemonising swarm. But they still follow them to the letter.