Some posts read to me as if some were very eager to find mistakes in @JeffBerlin 's point.
Seems counter-productive to me, since who can formulate a 100% bullet proof claim?
Sure, he is not 100% correct when claiming:
"The only proven way that guarantees musical improvement is to practice musical content on your instrument."
But does that mean the whole sentence is crap?
I see it rather as an invitation to think about
when does it make sense, and
what may be exceptions?
Stupidly simple example: does it make sense learning palm muting (PM) without musical content?
To me: 90% no, 10% yes.
Of course, it makes sense, especially in the beginning, to explore without musical content what PM can be; how it can be altered, what happens if I strike more or less strings, place my hand here and there, et cetera.
But that's only a fraction of what PM is about - the rest, I could learn much better (if not only) along musical content, as it involves thinking ahead during playing
when to PM, how, how
much, how strong, how long, when to switch back to normal playing; deciding / feeling how the bass may play while the guitar plays PM without compromising neither rhythm nor fatness of the mix.
And last but not least: Why PM at all?
Similarly for "theory" stuff as harmonic function; I can explore intervals/chord progressions like T-S-D and their effect on my feelings playfully on my own, pattern by pattern; except that I find them stupidly boring, one dimensional and useless - and with respect to the time spent on that exercise, I don't get enough in return on how to actually incorporate them in a song. Similar to learning to bow a violin with no violin nor bow, but a pencil and a roll of toilet paper; some do that, yet, to me it felt like a detour, a waste of time. A vilolinist needs a violin and a bow, and chords need music, IMHO - though, sometimes the chords or even a note might be music on its own; musical content, yet again.
So far, I find more examples within my memories that validate his claim than ones that falsify it.
Maybe that would be an idea for further discussion: Examples
P.S.: @matthewbrown This is not in response to your thread. I edited the first sentence to clarify that a bit, since it seemed as if I tried to say that your post reads as if you were eager to find mistakes in Berlin's claim - but that is not what I wanted to express.