When you say tie in rpg are you talking about john wildbow mccraes weaverdice/pactdice
(With reference to this post here.)
It's never just about one thing; SF/F authors awkwardly stapling some game mechanics to the side of their worldbuilding bible and calling it a tabletop RPG because people are more willing to pay money for it that way is a very common practice.
Authors who intended their tie-in tabletop RPG to double as a worldbuilding bible but ended up warping the worldbuilding all out of shape in order to cram it into a D&D-compatible framework is also more common than one would like; even A Song of Ice and Fire fell prey to that one at one point, if memory serves!













