You are correct that GN didn't measure exact temp for the APU. But at least his testing is as close as externally possible, from 1 layer of PCB, which can be taken into account of how hot it could be.
Also he did mention that some of the VRAMs, that they are actually very close to the board itself, and center of the chip (while explaining the location of thermal putty & backplate) - and says it's probably hotter inside. The only accurate temperature is only known to the manufacturers and thermal sensors inside the chips. Anything external - you have to take into account anything in between the heat source to the probe.
In case of the backplate - so you have "steel" back plate, which seems to be at least 1-2mm thick.
Below that, you may have the mounting steel (?) plate, and below that, you have that plastic and another metal plate then there is the PCB board.
GN's case - it's
Probe - PCB board - APU Basically 1 layer. Of course, the chip temperature varies differently on cpu and gpu section, so I think he put 2 separate probes into it. Then again, that's still an approximate temp - but the variable factors are greatly reduced.
This guy's case - it's more convoluted.
Probe - Steel backplate - possible gap - mounting plate - plastic - metal plate - pcb board - APU.
That's like 5-6 layers of varying degrees of layers with different thermal properties, that may or may not have been updated with this new sink.
Heavier mounts often require more heavy duty backing/support - and I wouldn't be surprised if they used slightly thinner, lighter duty metal on the back for new heat sink, and perhaps there's more gap betwen the mounting plate & the backplate? Perhaps OG PS5's backplate is hotter, because it's mounting plate is making more solid contact to the back plate?
At this point, it's just mere possiblity / variable that we don't really know... and that's why I'm not convinced with this "backplate" method.
Afterall, backplate for PS5 isn't meant for APU cooling - it's meant for VRAM and other parts cooling, which makes direct contact to the backplate via thermal putty. I do wish on my OG PS5 that they used somewhat different method to cool the VRAMs - but I'm pretty certain that they are within the specified thermal limits.