That said, if all you're doing is greenfield development I don't think it's actually possible to improve without third party input. A major goal of good code is maintainability, and that can only actually be known in hindsight.
I think the author means there's a correlation between the amount of code one writes and their progress.
For a more complete view, there's the often cited article The Role of Deliberate Practice in the Acquisition of Expert Performance[0] where the authors make a point that it's not just practice that makes perfect, but deliberate practice. They set to find and explain patterns exceptional people follow.
What you're talking about is what someone who says they have 20 years of experience but, in reality, they have 6 months of experience they've been repeating 40 times.
[0]: http://projects.ict.usc.edu/itw/gel/EricssonDeliberatePracti...
That is, if what they are writing works, despite having what could be seen as mistakes to a more disciplined practitioner, then what is the problem?
Consider, someone that has put together Ikea cabinets every week for the past year is probably hella fast at it by now. Can they replace a carpenter? Doubt it. Are they bad at what they do? Doubt it.
You make a good point, but imo in real life there is more nuance to it than just works/doesn't work. Which again I've seen with the same collegues: the software works, sort of, yes. Despite some annoyances and known bugs and corresponding workarounds it does what it's supposed to do. But it's oh so brittle. So while you could argue that it works so there is no problem, in fact there is: on every single change which has to be made half of the software falls apart and it takes insane amounts of patching to hold things together. Sure, afterwards it works again, but adding more of the same code, no matter how fast it was written, made things surely not better. I admit this is of course like the worst example and I'm sure there are indeed occasions where the code quality doesn't have that much of an impact. For instance especially in small applications/tools/extensions this is true.
Still, fun to think about. If you know of any studies, I'm interested.
This is a bit too simple: writing more of the same, in the same style, usually won't cut it and unfortunately I've seen people being stuck like that for years: always producing more code, without any improvement whatsoever and instead just repeating the same mistakes. Once those mistakes become a habit they are extra hard to get rid of. So actually writing more code can be worse :]
What you really should be doing is write more and from time to time different code, combined with learning through either reading others' or continuously asking yourself 'how can I improve this' followed by e.g. a trip to StackOverflow to find out.