Good for them. In many cases, getting good enough to do the job competently while doing it only at work is exactly how you get to the point of coding impressive stuff that is unrelated to work!

In fact, I've seen friends that decide to do something in a weekend side project that's super impressive, but it doesn't mean they're doing it every or most weekends. They're usually relaxing or living a balanced life, which in turn makes it possible for them to do their best work during the week.

Think about it this way. If a bodybuilder didn't take off enough days to rest in between really intense workouts and then prematurely plateaued at a certain point because they haven't respected the ideal rest/load balance, is that an obsession that enables or gets in the way of their fitness goals?

We shouldn't think that mental fitness is an exception.