All generalizations are false.
;-D
The "10,000 hour" or "10 year" mastery/expertise timescale is indeed arbitrary. Yes, there are other factors which significantly impact attaining mastery, many of which are mentioned in this thread. Even more detail can be found
here.
Even so, to dismiss it as "tosh" is irresponsible. It's a marker along the road to mastery.
Also, chalk me up as another who has a very definite idea of what constitutes "practice". Just playing for fun is not practice. Ever. At all. Performance is not practice. Ever. At all.
Practice must have structure. It must have clearly-defined near-term and long-term goals. For example, sitting down to work double-bass speed through repetition of specific, planned exercises is practice which leads to mastery. Sitting down to learn Metallica's "One" is not.
Can you get better by jamming or playing along to recorded material? Sure. But - surely you saw that coming - you'll get better faster if you stop being random and start planning how you're going to improve.
Of course, my definition of practice is narrow, as it only encompasses the development of technical skill. It's internal. In order to develop as a musician you do need to get out of your basement and play with people. To be external. Part of mastery is learning how to put your technical expertise to practical use. As musicians that means playing with other people. Only then can you develop sensitivity and communication.