I think JC might not respect the Russo bros as filmmakers. From his POV, they're jus a wheel in the marvel machine, with very little creative control. That may not be the reality, but there's this perception that the MCU is like a factory, with a lot films having a similar tone, look and feel, and even sharing the same second unit directors taking care of certain aspects of the films. It's probably why Cameron doesn't view the Russo bros on the same level as Spielberg, Lucas or himself. Probably why he congratulated Feige instead too.