I disagree. Resolution is not measured by the number of pixels.
So i can say a FF sensor with 300 pixels and a FF sensor with 21 MegaPixels have the same resolution if its not about the pixels.
The camera sensors are made out of pixels so how u wanna measure resolution in another way if the camera sees the world all in pixels and nothing else?!
I believe his point is MP alone does not provide a clear cut answer to how much detail is resolved by a camera. For instance, compare a picture taken with a perfect lens and a foveon sensor to a picture taken with that same lens and a bayer sensor with a heavy AA filter - even if they have the same MP, there will be a difference in what's resolved. Also, how about a 36MP D800 with a Nikon 500/4 compared to lowly 22MP 5D3 and a Canon 500/4 - The Canon system resolves more detail despite a lower number of MP.
Obviously it'd take a phenomenally bad lens (or AF system) to equal the gap between a 21 million pixel sensor and a 300 pixel sensor. The number of MP a sensor has sets a ceiling on the resolving power of a camera system, but many other parts of the system can work against realising that theoretical maximum. Like anything, its all down to the weakest link in the chain.