But during the 70's and 80's a lot of manufacturers stopped printing schematics in response to clones appearing which used a circuit that was close (but not exactly like) theirs and sold at a deep discount[1].

Early computer board manufacturers used programmable logic to create special "nodes" within the system which obscured the functioning of things such that to clone them you would have to hire an engineer to understand the design and re-design it. Generally that was more effort than most of the cloner companies were willing to go through. (apparently if they had their own engineers that could understand the circuit they didn't need to clone, they could design their own)

FPGAs get you at least two layers of obfuscation, both the bit stream is encrypted and "decompiling" from the place and route configuration bits is not straight forward at all. Folks who have gone down the path of trying to figure this stuff out (as the OP is possibly doing) discover that changing a routing bit way in the front changes a bunch of other bits later but in very subtle ways. And routing in an FPGA is a great way of screwing up function, when you do timing analysis if you have to run at 200Mhz you have to make sure your setup and hold times are accurate to within a few nanoseconds across temperature and voltage differences. FPGA vendors tout this as a 'design security' feature some times.

Decoding this data is a lot more challenging than reverse engineering an image format.

[1] And yes you can argue this is just business, and it is, and this is just "business defense."