The problem with the 10X label is that it models the productivity as linear relationship, when it's not, as you allude to.

A bad programmer can do simple tasks nearly as fast as a good programmer.

But at a higher level of difficulty that difference might be 10X. Hell, it might even be infinite.

For example, the best NFL QBs will win 10X as many games as the 3rd string backups. But you wouldn't call them 10X better. In fact they are probably only 10% worse than the elite QBs in their completition percentage.

Or if you raised the height of the hurdles 4 inches in hurdle racing, some of the racers wouldn't be able to jump over it any longer.