Irrelevant; I don't know what this means; it would be falsified by showing it has no correlations to things.
> g is particularly bad at explaining musical talent
I don't know why you'd expect it to correlate to music talent any more than it would correlate to visual artistic talent or bowling talent. I don't think those are things that have traditionally been associated with intelligence. The things that g correlates with are the things that people have associated with intelligence for thousands of years; we all have an intuitive sense of what it is generally referring to.
> My issues...
I mean yeah, any multivariate test reduced to one number loses information; the SAT score is meaningful for many purposes even if it may be produced by heavily skewed subsection scores. The other thing is that you're picking an arbitrarily small distribution, height. The 95%ile in shoe size is even fewer inches larger than its mean than height is! But also the 95%ile in chess elo will win 43/44 games against the mean elo player. There is very good reason to believe that the 130IQ child is meaningfully different from the 100IQ child: that is the entire basis of gifted educational programs that have shown through research for many decades that the former child has very different needs than the latter. Also if you've ever spent any time around kids/students, especially tutoring them, it's pretty incredibly obvious that the gaps in ability are vast. It's really not a matter of inches here, it's a matter of literal years of schooling being wasted by putting gifted children through standard public school tracks.
It doesn't matter that it's a relative ranking. It matters that the gifted kid has different needs than the non gifted kids who have different needs than the remedial/special ed kids. Grouping into 3 is an arbitrary thing, but the difference exists. We can't pinpoint on a spectrum when a hue changes its label, but we can all agree that between either extreme end there is A and there is B.
I am concerned about this because my own education was extremely painful due to this sort of debate as a 'gifted' kid. If my schools had had advanced tracks for all subjects, not just a few, I'm pretty sure I would not have despised school so much or had as many teenage mental health issues. It is so soul crushing being trapped in a classroom every day with essentially zero learning or interest happening, being forced to performatively "pay attention" or "take notes" like a drone, etc. I get riled up when I hear opinions that may lead to support for abolishing gifted programs.
The term "gifted" is useful for helping and classifying children. Maybe you are saying to replace it with more granular words, like "gifted in math, standard in english" or something? That's fine and generally what happens in practice, but it's just going to be a recursive debate: "How can we really say you're gifted in math? Math contains a lot of subjects..." We have enough problems already trying to get modern school districts to understand differences in ability exist at all.