I think the key here is that you only deliberately practice things that you really, really want. Things that you want so bad that you obsess over them and you wake up realizing you've been dreaming about them. I quit a comfortable software job to take about 9 months off to devote to piano recently. Every free moment I get, I'm throwing myself in the waters. It's really hard to push through the painful points of learning stuff if you're not obsessed.
I deliberately practice a lot of things that I'm not obsessed with, it's really useful when I'm overwhelmed with a new field (e.g. learning a new framework), to isolate a small bit I want to master and try several different ways of doing it, and document my findings.
That allows me to not see what I'm trying to learn as an obstacle to what I'm trying to achieve. It removes a lot of frustration, since that shifts my view of the task from being an obstacle to being the goal itself.
I'm not sure I would say that I focus on deliberately practicing the same few skills as often as the author of the article does, though. Usually, even for the few skills that I have deliberately practiced for a long time, my focus has been on exploring variations or adaptation to new context. That is because I aim for adaptability rather than having muscle memory of a skill.
I think the real key to top performance is to learn to get over the aches of practicing and training yourself to get engrossed in a range of subjects. Piano is good practice but I usually recommend something more dopamine efficient.
