This is bollocks, because frame rate is dependent on the CPU and that of the PS5 is essentially the same as that of the XSX but is slower by 100MHz; however, its unified cache, which the XSX's CPU doesn't have, should compensate for its slightly slower clock speed due to its decreased latency. Hence, the lack of native 4K and 120Hz is a result of the developers being unwilling to implement them.
As for the GPU of the PS5 being less powerful than that of the XSX, it's less powerful by only 18% at most; hence, in order to enable native 4K and 120Hz, the graphical quality of the PS5 would simply have to be tweaked a bit (e.g. slightly lower resolution textures and or slightly less post processing, etc).