Right, but it's harder to detect subtle manipulation. It only takes a one bit error to reduce the key space by half.
It's possible to write obfuscated assembly, of course, but it sticks out as obfuscated.
To further get your head around the lack of cover compiled code would give a backdoor, it's worth knowing that Hopper will give you serviceable C-ish code from x86_64 for... let me check here... fifty nine bucks.
(It's possible that my only message with that last graf is "Hopper is great").
if some specific set of registers is set a certain way followed by a very specific series of commands, there is no practical way to prove that the hardware is doing what it should.
Meanwhile: if you can't trust the hardware, you can't trust C code either.