If I could place the RX100 sensor in A500 for bird shooting, .........

No, he's saying that if you use an f/2.8 lens on a 2.7x crop camera, the total number of photons captured by the sensor will be the same as those of a f/4.2 lens on a FF camera. Which makes sense, because out of all the photons being captured by the f/2.8 lens, you're throwing away any which doesn't hit the part the "cropped sensor" is "cropped from", to put it some way, and the ratios between the areas it's covering and the area you're actually capturing is the same 2.7x.

The problem, of course, is that this figure is only useful for calculating stuff such as the quantum efficiency of a given sensor technology, but for actual photography f/2.8 is f/2.8 is f/2.8 and ISO100 is ISO100 is ISO100; that consistency is in fact a prime reason behind why ISO was standardized as such, and if the total number of photons captured mattered, then it's likely the ISO figure would've had a factor integrated into it to compensate accordingly.

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