I've always read 10x as an order of magnitude better, not necessarily 10x faster. 3 sigma is probably better terminology though.
The gambit is that a mediocre programmer would take 10x as long to write the same code as the hypothetical 10x dev. But that’s not what a mediocre or even average dev does: they solve the problem in an entirely different (and, for purposes of this argument, less optimal) manner in some spam of time that may or may not be longer than a qualified hacker or a theoretical 10x dev. Almost all CS problems have multiple solutions, so you’re not likely to find one where there is only one solution and it takes one guy x and the other guy 10x as long.