i do my best work when i'm in the zone and it has nothing to do with whether the work itself is challenging or not. in fact, the more challenging something is, the easier it is for me to stay focused if i feel like i'm making progress
The claim boils down to this: you will do your best work in a state of flow, but you will improve your capabilities faster outside of it. Flow is great for productivity but not for changing how you do things.
At a handwavy level at least, it seems to be true in my experience. It is certainly true that not all practice is equivalent, and you can put a lot of hours into "practicing" something without making any real progress in your skills.
flow usually equates to deliberate practice.
Deliberate practice is never relaxed - but flow can be.
To reference the OP, there was discussion of intentionally focusing on and making the difficult parts of a piece harder as a form of practice. That doesn't bring flow, but allows the flow state later when you play the piece "for real". I think this generalizes well.
This isn't rocket science, and it is something professionals are taught to do.
But I'm also not trying to play extremely fast. If my goal was recital rather than composition, I would use it more. My recordings[1] are mostly total improv, free jazz, with a vocals track (later deleted) that states what chord changes are coming up, and then improvise over the first instrumental track while listening to the chord announcements.
All the exercises I received from Charlie forced me to be consciously engaged in the act of playing- never outsourcing things to muscle memory.
I think that largely this was to avoid falling into mechanically playing over changes and to be actively engaged in thinking about the music at the moment it was happening and responding to it creatively.
Flow (as defined by Mihály Csíkszentmihályi) is between frustration and boredom because either the activity is too difficult or too easy.
Going at something for 3 times half an hour with hours in between is more effective than going for hours straight. And this kind of focused time method also works well for programming. With programming it is harder to decide where to put the break.
Breaks don’t mean you have to do nothing, you just have to do something that is mentally different. When I take a programming break, making music, soldering or reading is quite a good programming break.