Do people really question the existence of "10x engineers". It follows from Pareto distribution that some people are just exceptionally productive. That doesn't mean that its good or beneficial (depending on what the scope is). That also doesn't mean that its good to only have them, because they often carry their own baggage.
It is worth noting that the original 10x was a reference to the best engineer being 10x more productive than the worst engineer. Not 10x more productive than the average engineer, no the median engineer.
I question the usefulness of that way of thinking. 10x isn't an unchanging skill level, it's comparative performance over a period of time, and that extremely high level of performance likely has many contributing factors. Saying "oh, John is a 10x engineer" stops you from exploring how John is doing ten times the work of his colleagues.
There are good answers, like John is an expert in the specific technology you're using and he was hired through a meet-up you sponsor; there are bad answers, like John has found a way to game the ticket system; and there are debatable answers, like John's team has a high support burden which he never helps with. If you insist on seeing him as a 10x developer through innate skill or divine fiat, then you're not going to look for the root cause of why he's doing more work than everyone else.
People disagree on what it looks like. If you think it’s an engineer who types 10x more lines of code, or makes 10x less bugs, there is someone ready to argue that person doesn’t exist or is actually a boat anchor.
Once you define it as an engineer who ships 10x as much business impact (ships fewer bugs, builds the right features, etc), it’s less contested but also less glamorous and harder to measure. In fact you start to develop an inkling that such people may not even be the same coders you once idolized.
