There’s also macroscopic single cells, as well as prokaryote colonies that are macroscopic in total, but some things just fall outside our scope.
The gameplay reason here is that only acquiring 20 cells is very easily done, so if we don’t enforce it, people don’t actually play around with the stage’s mechanics. So that would require other transition demands, or make growing your number of cells much slower.
By necessity we have a jump from 20 cells to a 1 cm organism. So our Multicellular Stage has to represent in that 20 cell frame things that are necessary to get to that 1 cm scale. So a Multicellular Species of 20 cells represents something much larger IRL.
Looking back at this again, the axon actually cannot be designed to do what Deus describes here (a “general” signalling mechanic), because an axon represents real nervous cells. Like we talk about frequently, we want nervous systems and Aware status mechanically mostly restricted to motile heterotrophic species. So the axon is something that should be something that’s only useful for motile species, not every species.