10x engineers do not work 10x harder or produce 10x code. What they do is write code that is 10x better, in that it has few problems, few design errors, does not have to be rewritten (at great cost), is more adaptable, maintainable, etc. They make the right decisions up front about the approach to make.
For example, they can select the right approach to concurrency for a particular problem, rather than spending much time later chasing concurrency bugs.
But you are likely aware that this type of 10x engineer is not who the vast majority of corporate managers and "non-technical" start-up founders are thinking of when they describe a 10x engineer. For them, a 10x engineer is simply one who produces code 10x as fast as the other engineers.
In fact, simply writing code 10x as fast as usual is not a difficult task for a competent developer to accomplish. With significant overtime, and reckless abandon, any skilled developer can churn out poorly thought-out code at a rate roughly 10x greater than the rate at which a skilled developer can write well-designed, tested, documented, refactored code.
This is why we have so many "10x" engineers, and so much technical debt.
For example, they can select the right approach to concurrency for a particular problem, rather than spending much time later chasing concurrency bugs.