As I understood the article, they made no assumptions abut connectedness. They basically tried all combinations of 4 galaxies out of their sample set, and counted the handedness of all those. some clever maths required to deal with the combinatorial explosion

Yep. The key here is that if you choose four arbitrary galaxies from a set, there's two possibilities -- their positions in space are close enough that there's interesting things to be learned about their formation; or they're not. In the latter case, there's a strong reason to expect no bias to the tetrahedral side lengths; in the former case, there may or may not be. If you then look at all possible combinations of four galaxies -- including all the four-tuples that are "real" in 3D space, but also all the four-tuples that aren't -- the average of "no bias" and "interesting bias" will be "smaller, but interesting bias."

Which is why, once a bias is found, figuring out it's p value is non-trivial...