Controverstial. Personally I think this is a case of Lies, Damn Lies and Statistics. The SRAM 10 tooth sprocket is significantly less efficient than the 11 tooth, so if you compare riding in the highest gear then the SRAM may be 5 watts less efficient (athlough not sure if I believe that either). However, I cannot see any mechanical reason, why a SRAM setup is less efficient
A lot of the Trek pro riders don’t like SRAM but kind of don’t have the choice. Now retired riders were pretty vocal about it.