You're right. That's precisely the problem. The "Nx Engineer" is catchy and we all know "what it means" when we first hear it. We most likely relate it with an early experience when we were juniors and a senior developer pair programmed with us. For me it was mind-blowing. So we can relate to it.

But the issue is the ambiguity. The multiplier is there, but not the unit/metric being measured. So it's just a shorthand for "someone exceptionally good" as said by a sibling comment.