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This is false. If you want to strum some chords, then, yes. Guitar's difficulty curve starts out very gentle. But then it quickly gets really, really steep. I self taught classical guitar for a couple of years (after already having played violin for many years), and it's easy to get yourself to the point where it's hard to advance without a teacher. And the harder you push on that knee in the curve by yourself, the more time you'll probably have to spend unlearning bad habits.

Piano's difficulty curve is more even. It starts steeper than guitar, and has a long stretch at about that same slope, before it starts getting steep, too. Compare that to violin which starts basically straight up, but once you get past that initial cliff is fairly gentle for a long, long way.

I have come to dislike this method. It gets the student focused on looking at the fingerboard and using the small lateral muscles in their fingers to try to reach for the tape rather than finding the large muscle positioning and biomechanics that produces in tune fingerings.

Hold up your hand with fingers straight. Try spreading your fingers and then pulling them together fast. They kind of "pop" between the two positions, and if you look at the spread position, they're basically the same distance apart. So if you can arrange your arm so that spread drops an in-tune whole step on the fingerboard and together drops an in-tune half step, you will play in tune.

You do that by rotating your elbow under the instrument. The farther toward your back the elbow is, the more your hand is angled from the neck, and the closer the spacing between where your fingers fall on the fingerboard. The more your push your elbow forward, the more your fingers fall farther apart on the strings. There's an in-tune position for each of the four strings, and kind of inbetweens for double stops. Training your gross muscle memory to find that position is vastly easier than searching for tapes, you don't get in the habit of looking at the fingerboard (because there's nothing there that makes a difference), and you don't get slowed down later because your lateral muscles in your fingers are tense and making your fingers move slower.

The funny thing is that I think this was described in Geminiani's treatise, which was the first manual of violin playing ever.

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Come and do your reps!

Fundamentally, technique differs a lot based on size of the player. Ricci vs Perlman vs Shaham vs Midori vs Hahn vs Paganini, they all make the technique looks effortless. And the a lot reason why is because they've spent a lot of time removing bad tension from their playing.

While elbow positioning does matter, it is more a function of string and third vs fifth hand position. The fingerboard is curved, not flat, and you want the finger to land normal to the board for any given string.

Look for the work of Sol Babitz, or if you can't find it, Borivoj Martinic-Jercic's forward to his exercises.

For me, what helped the most was removing the shoulder rest during practice. You become accutely aware of the the delicate balancing act that needs to occur, and the forces you put on the instrument. With time, you can develop balance in the left hand and achieve better dexterity than before. For anyone with sufficiently long neck, you can no longer rely on continual holding, and need to support the instrument (in fixed position sections) via the thumb. This encourages you to release pressure on the digits as you play. :-)

But like OP said, they're happy so why mess with what works for them. :-)

P.S. what rep are you working on? Feel free to drop me an email if you're interested in discussing more.