Well, Ritchie is normally credited with the creation of C and Thompson with its predecessor B. You're probably right, with the two coworkers working closely together on Unix Thompson probably made a lot of direct contributions to early C. But it was Ritchie that took C as his project and shepherded it through its growth and standardization.
Certainly Dennis Ritchie is the primary author of C, but given the very close historical relationship between C and unix, we know that there was a tight synergistic evolution that shaped each in relation to the other. As an example, very early C didn't have structs, but Thompson clearly needed a bit more powerful abstractions for some of the work on converting unix into C from pdp assembler, so Ritchie added them.
I was really just quibbling over definitions and connotations, when I hear of an "indirect" involvement I think of something very different and much more remote than the deeply intertwined stories of unix and C and Thompson and Ritchie at Bell Labs in the 69-74 era.
