I can see where it could be seen as separatism, but it really all boils down to organization. Piecewise's game is so large it takes several ongoing threads, and even more will spring up as the current group fragments further, not to mention the lack of a dedicated out-of-character thread that some have been proposing.
Consider Roll To Dodge itself. It's a forum game genre, but eventually there were too many of them - the entire front page of FG&RP was nothing but RTDs. To organize them better and to allow players of other games - that might not like them, or care about them - they were moved off to a separate forum. They aren't "better", and they weren't moved just for the sake of making a new forum - they were just too many, and they were interfering with what others were doing.
Let's put it another way. Einsteinian Roulette is currently spread across three threads. Ship thread, Mission thread, Secondary mission thread. Like it or not, it's going to need at least two more once the current story arc is complete. Tinker, and Hephaestus mission threads. Maybe an OOC thread. They are, barring brief hiatuses and Sundays, all going to be updated daily, with the entire crew of thirty-something players and the GM all posting in them. That's five/six threads continuously hovering on the first page of the RTD forum. For just one game. It may not seem like much, in the overall thread count, but that's the question then - are you more okay with one game having its five/six threads constantly hovering near the top than with making a separate subforum to put that game, and perhaps games like it if they appear, into?