This is not that concept. That concept is no one but us can technically complete the exploit. Technical feasibility in that you need a supercomputer to do it, not protecting a backdoor with the normal cia triad
If they determine the vulnerability is only exploitable by the NSA for reasons
such as computational resources, budget, or skill set, they label it as NOBUS
and will not move to patch it, but rather leave it open to exploit against current
or future targets.
I'd like to rule-out some state actors by the mere heartbreaking act of socially engineering and targeting a person in pain.
Not even considering the original maintainer may be a US citizen (idk).
If they didn't have the key though, then yeah it doesn't fit. Unless they can walk through ssh key security anyway. :)