Here it is. The reasoning seems to make sense to me. The key is you have to take the lens and DOF into consideration when doing the comparison. You must make the comparison based on photos that show the same image (same DOF, same perspective, same field of view, same exposure).
Let say you took an image on an aps-c camera with a 300mm lens at f5.6. On a FF camera, you will have to use a 450mm lens and use f8 with the same shutter speed and one stop higher in ISO to get an identical image with the same DOF, FOV, and perspective. So on the lens you lost a stop of light and must shoot one stop higher in ISO. The 1 stop ISO sensitivity gained due to larger sensor size for the FF is negated by the fact you will have to shoot 1 stop higher in ISO to get the same photo - so it is a wash. Therefore going from APS-C to FF using the same sensor technology is not going to give you higher net low light sensitivity gain when you also take the lens into consideration.
To look at this differently, yes, on the 12MP FF, the IQ (noise level) from shots taken at ISO 3200 is the same as shots taken with the 12MP APS-C at ISO 1600. So I think it is "better" and I must have one. But I will find that I will have to shoot at ISO 3200 to get the same image (same DOF, FOV, perspective) that was taken at ISO 1600 on the APS-C. So there is no real gain and I am now disappointed and wondering why I did not buy the 24MP FF (also no net ISO gain but you at least get more pixels) one instead or even bought it in the first place. Note that I may be disappointed if I was only looking for better ISO performance. I may not be disappointed if I was looking to get a wider view using my existing wide-angle lens - assuming acceptable lens edge performance.
Note that the above is based on theory which I think is sound. But the manufacturer may have better sensor technology and decides to use it only on the FF first. So in practice, you could indeed get some low light advantages until the manufacturer start using the same technology on the APS-C. Doing valid real world comparison testing is key before buying.
https://www.pentaxforums.com/forums/pentax-dslr-discussion/71896-low-noise-be...uals-zero.html