If the metal isn’t there it won’t show up in device tree because it physically isn’t connected on the die. So device tree as you keep pointing out cannot tell you if it’s same die or not. Also the device can be there and not be in device tree meaning the devices memory location was omitted from device tree when the device tree blob was built.
It may well be that they have different die. But that’s usually an expensive way to do things. Usually you want downgrade paths for devices that don’t yield.
They do have downgrade paths for chips that don't yield: disabling some CPU or GPU cores, but not half the chip. That's why Apple sells those variants. Also, the M1 Max die physically doesn't fit on the substrate used for the M1 Pro.
Source: over one year working on reverse engineering this precise platform almost full time.
Also, if you've spent 23 years in the semiconductor industry, I have no idea where you're getting the "not connecting metal" story. Nobody does that. How would you even do that for a yield issue? That doesn't make any sense. Chips that have failed bits get the broken parts marked bad via eFuses after production and the initialization logic or bootloader will then read the fuses and power/clock gate those bits and lock them in that state, via existing isolation/gate logic. That's how the entire industry does it. Metal patches are for fixing design bugs in a respin and stuff like that, not for turning things off in a finished chip. And you certainly wouldn't make a change in the line to the metal to disable half a chip from the get go. That's just throwing silicon away for no reason, why wouldn't you try building the full thing first and seeing what works? The entire concept makes no sense.
It may well be that they have different die. But that’s usually an expensive way to do things. Usually you want downgrade paths for devices that don’t yield.
Source: 23 years in semiconductor industry.