The problem is when there is an expectation that as a developer you're somehow inferior if you don't code in your free time.
Insofar as diversity of experience matter, coding less and on a narrower array of projects does make you inferior. So, yeah, I’d expect developers who don’t code outside of work, all other things being equal, to be inferior to those who do.
I’m curious what, other than simple self-interest of those who don't code outside of work, would lead one to argue that that isn't the case.
Additionally, doing things other than coding also provides diversity of experience which may be relevant and/or offer a wider perspective.
So, I don’t think you’ve really made your case here.
It might be (I don't think it's practically orthogonal though it's obviously not strictly linked), I should have mentioned both quantity and diversity of experience (and, in fact, thought I had; I must have dropped part of intended edit.)
I can spend W + H time making X widgets, granting more quantity in the same scope or W making X widgets and H making Y (or Y0...Y∞) systems, for increased diversity with the same quantity in X.
That is not an absolute. Being a developer is not just about writing heaps of code.
Maybe the developer who volunteers at a local community centre is better at dealing with people. Sure, they might not code as much, but they might be better at teasing requirements out of BAs.
Or it could be as simple as the developer who does their work during the day, goes home, cooks a meal, plays with their kids/socialises with their friends comes in the next day rested and ready to focus on their work, instead of trying to thinking about that problem in their personal project.
Or it could be that the guy who codes 24/7 is a total rockstar. But maybe working on that pet ML project in Tensorflow at home isn't really helping you write that REST endpoint in ASP.net at work.
There's no guarentee either way.
Distracting one's mind from the immediate task can also be beneficial for the end result.
All this does not make me inferior. Rather the other way around. Luckily I've been my own boss for the last 20 years so I do not have to explain how I do things. I am only responsible for end results /time frame and overall cost.
Granted there are situations when loads of code needs to be written that does not require much thinking. For this I usually hire subcontractors. Sometimes do it myself but it does not come very often.
So before I got that first job, I focused on learning general things to raise my ceiling, so I studied text books. OS, DS and Algos, Calculus, DB Implementation, and others.
Now that I have my job, I’ve focused on the areas that i am tasked with in my day job, like DB deep diving and Statistics and Linear Algebra.
I always felt that any side project I would focus on would impose too high an opportunity cost because I fealt I was learning better things.
I am still growing and will continue to grow as long as I’m in this field but my github has nothing noteworthy to show off.
That said, I believe your point is correct in the abstract, namely, that Devs who spend time outside of the office to level up will be better than those that don’t All Other Things Being The Same, I’m just saying that side coding projects isn’t the only instantiation of that activity