Is there another place in the NEC or IRC that references the connector requirements when installing cables in conduit sleeves? I found 312.5(C) and I'm thinking there is another code reference that says you don't need a clamp inside the box to secure the cable to the enclousure.

You sort of get there with 334.15(B),300.10 ex1 if only by omission.
If you are coming out of a box, exposed to some degree of physical damage it is normal to sleeve cables in EMT or RNC up to a safe height, where it gets secured upon exiting the raceway.