I'm part of the project that produced this board.
I am definitely, yes, old enough to remember the Transputer. And I hacked artificial life models on the MasPar in the early 90's, which had an architecture in some ways similar to the Connection Machine.
Although the IXM is indeed 'embarrassingly suitable' for assembly into planar grids, it certainly isn't restricted to that. With right angle headers, for example, it's easy to make shapes likes rings and cubes and so forth.
When the global computational geometry of a machine is fixed at design time, before the ultimate task is known, routing can easily become a major problem. And general routing is hard. Maybe too hard.
But part of exploring modular systems in the 'physical computation' space is trying to figure out ways to make the geometry of the particular computer you build better fit the behavior you're implementing, which can help ease the general purpose routing
And if one really gets into a corner, well, ribbon cable is cheap.