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.