The advice in the article is probably right for people who want to be the very best, but probably wrong for most people who might read it.
But what if your goal is to be one of the greatest pianist alive? Then we have to dissect what is "not fun". There are moments of neutral practice, or "monotony", which is what I think the article touches on. The idea that sometimes, practice can feel more like a chore than a pleasure. The article speaks about in this context, you have to push through the negative (or apathy) to continue practicing.
Last year, I had the privilege to attend a seminar in Aikido where two individuals were awarded the rank of godan (5th degree black belt). In our organization, the minimal amount of time required to be eligible for the rank is just over 18 years (and the requirements are increasing next year). One of the individuals spoke about how Aikido and training was much like a marriage - sometimes you enjoy coming to class, sometimes you hate it, and sometimes it's neither feeling; however, much like a marriage, good times or bad, you have to continue working on it.
I've spoken on here before as well about the element of discipline/perseverance/grit in terms of improving oneself. Angela Duckworth has studied the effects of grit across multiple fields, one in particular was grit in cadets at West Point. Higher levels of grit were more likely to complete the program and good leaders were more likely to have higher grit scores.
"Fun" is still a loose, ill-defined term. You can have fun being pursuing a task for leisure or for mastery.
After playing 20 years myself, I must say I'm not nearly as obsessed as the 5 or so pianists I know who play for a living. I play folk and pop, classical I'd say is merely a hobby to me, therefore I'm a scientist in my day job.
The saying goes “practice makes permanent”.
Some stuff remains for sure, but you lose a lot after a couple of years of not playing.
You're speaking in very strong terms, as if we really understood how people become good musicans, but I think we really don't understand it and people take many different paths.
(My background: I started on guitar at about ten, switched to mandolin at about twenty, have always dabbled in other instruments, and at this point my main one is probably piano. I play in two contra dance bands, https://kingfisherband.com and https://freeraisins.com , and mostly play for dancing.)
I hate going to the gym. I just don't like it, and that makes it hard to get good at it. I can make myself go, that lasts 1 to 3 months, I eventually hurt myself because I was to cheap to invest the resources necessary to learn proper form, I take a couple weeks off, lose all my progress, and get stuck on the fact that the same cycle is gonna have the same outcome, and I just stop going.
I love going to handstand classes. I see the same small group of people most classes, I know most of them by name, and for some reason I like working towards handstands. It have been taking these classes for 6 months, and my progress is slow for reasons (discipline, bi-lateral radial head replacement, weak starting point), but I love going. I spend the time necessary to learn the body awareness I need, and I get private lessons when I get stuck on learning something harder.
I highly recommend not bothering with exercise choices you think you have to do, and seeking those you wanna do.