This was discussed when Ghidra was first open sourced. To the best of my knowledge, nobody's found an NSA backdoor in Ghidra.

Seems like it would be of limited value to backdoor a program like Ghidra. Might be useful in identifying security researchers, except that it's also the kind of program that will often be running on disconnected systems with little valuable data beyond whatever is being disassembled.

Without providing any proof that either this or SELinux is backdoored.

Well it’s open source, so you can check in principle. I would imagine there’s some fame and notoriety in discovering that.