Figuring out the block diagram of a chip is very different from mapping out the physical location of all the busses and gates so that you can splice new circuitry in without damaging the existing functionality.
Reverse engineering for compatibility does not give you an insight to any backdoors there, just as even having the full source code for a system does not trivially reveal all the backdoors or bugs it has.
If this were true, then the Apple M1 would never be as reverse engineered as it is now.