You misunderstand; if you can't even decrypt the binary, you can't disassemble, much less run a decompiler over it.
As someone who has done quite a bit of reverse engineering work, I have no idea how I'd identify and isolate a vulnerability found by fuzzing without the ability to even look at the machine code.
If it runs, it has to be decrypted (at a current level of cryptography); at most it is obfuscated and the access is blocked by some hardware tricks which may be costly to circumvent, but there is nothing fundamental stopping you.
As someone who has done quite a bit of reverse engineering work, I have no idea how I'd identify and isolate a vulnerability found by fuzzing without the ability to even look at the machine code.