The "10,000 hours to expert performance" notion originated in Ericsson 1993 (http://projects.ict.usc.edu/itw/gel/EricssonDeliberatePracti...) , which defines a very specific term, "deliberate practice" -- "In contrast to play, deliberate practice is a highly structured activity, the explicit goal of which is to improve performance. Specific tasks are invented to overcome weaknesses, and performance is carefully monitored to provide cues for ways to improve it further. We claim that deliberate practice requires effort and is not inherently enjoyable." Ericsson adds more detail in the full paper.
It's not just "are you playing piano for X hours per day".
The paper in this post mentions deliberate practice in its review of the literature, but then all of a sudden and without explanation reverts to the more generic term "practice" for all of its contributions, which appears to be defined as "how many hours did you play your instrument".
Note that "play" and "practice" are explicitly disambiguated in Ericsson.
I have a hard time believing that the authors are unaware of this distinction, especially considering that they heavily reference Ericsson.