That's a bit of a strawman. My full comment is that being familiar with your language of choice, and having a deep understanding of a problem means that can produce a solution faster than someone that doesn't. I used the phrase 'churn out', I could have simply said 'produce'.
I've worked in the industry for almost 30 years, and I've worked with a lot of people in that time. Those that you might qualify as '10x', all have had both of the qualities I mention.
I would not expect any of those people to switch languages and fields and still be a '10x' developer.