Thanks for asking. I'm usually a stickler for any self-improvement advice being transparent about how it was tested, if at all. I should do the same here on my personal blog.
This is a theory based on personal experience. For example when I say half of people will exceed anything they've done before that really is a rough estimate based on having given lots of people way more room than they are used to. I have no idea, yet, how to predict who will succeed in that situation. But it seems to me it ends up being less about talent and more about preference. Nobody really knows themself well enough to know if they will prefer one or the other.
The other 1x way, btw, has a lot of benefits, namely certainty and positive reinforcement. If you know you can do a 1x job well, get paid well for it and get regular positive reviews from your boss and have flexibility to job hop to many other similar 1x jobs, why would you want to do a 10x job?
A concrete example was realizing our iOS engineer was also an excellent visual designer. It "10x'd" the speed of development because you could just give him a problem and he'd do both sides of it. But where else are there jobs where the iOS developer is also your lead designer?