That's the point. If you don't have a metric that says "Alice is 10x better than Bob" and you don't specify a task she's better at, then what you really mean is "Alice is
on average and
by some unknown factor much better than Bob
on the tasks she performs".
Framing it as though you have a rigorous comparison between them will mislead you into thinking about engineers as if they were fungible, like diesel generators, which is a very silly way to think about a divide-and-conquer field like software engineering where engineers develop different levels of expertise in different things.
If you're suggesting there is some underlying factor, like generalised intelligence, I'm not arguing with that! I strongly disagree that factor has anything to do with the number 10, or that engineers fall into discrete 1x and 10x camps, though. I think the phrase "10x engineer" leads to logical fallacies about performance.
