Right....Primus doesn't seem to have worshipers in the traditional sense. Perhaps all Modrons would be considered his followers, but I don't think that's really accurate. They're almost more like an extension of him.
Same with the Lady of Pain. She won't allow people to worship her. Yet she can block the highest of deities from entering Sigil. Certainly she's worth inclusion in discussions of incredibly powerful planar beings...but she doesn't meet most of the criteria associated with deities. Neither do many other Powers.
So Primus might be more akin to the Lady of Pain, an Overgod.