No. I think it multifaceted.
Some QB's excel with or without having a private QB coach, it really just depends on the kid.
Contrary to popular belief, a lot of Black QB's do have independent QB coaches, Deshaun Watson, Kyler Murray, Teddy Bridgewater, Lamar Jackson, Jameis Winston, Geno Smith, Dwayne Haskins, Justin Fields, **** even going back to Cam Newton, they all had private QB coaches.
QB development to me has more to do with the process of Youth/Little league football & how kids are developed going into HS than anything. In regions like CA, TX & AZ, they run College level Spread/Air Raid style offenses starting in Middle school, so by the time they hit HS & College they're already well versed in the play calling & operational aspects of QB'ing.
The Private QB industry does help a great bit because it gives kids good teaching in terms of refining their technique & strengthening their throwing mechanics, but just as many kids who have personal QB coaches still have a high bust rate at the upper levels of football because ultimately traits & skills that can't be taught are what win out.
From a regional standpoint, being a multi-sport athlete at QB plays a huge part in development as well, the QB's who played Baseball, Basketball & Football growing up usually tend to excel at the next level. Particularly in TX the Baseball culture is just as strong as the Football culture down there, so just about every single TX QB grew up playing Baseball as well & due to the muscle activation in the shoulder & arm they tend to have more natural arm strength at an earlier age & develop faster at QB in High School. The Baseball culture is huge in South FL too & produces the highest volume & some of the best Baseball players in the world, but most talented South FL Baseball players usually specialize & don't play Football, they play Baseball only, so the kids with super strong arms (particularly in South FL) aren't mult-sport athletes.
It also has to do with school funding, a lot of schools down in TX have private funding with big time donation boosters who spend millions of dollars on weight room facilities & college style Football fields, so those kids are getting as close to professional training from their Freshman year until they graduate. South FL public schools are the exact opposite, the weight training programs from most kids is subpar at best & they have to rely on personal & individual workouts to get good training.
It also has a lot to do with Coaching, up until a few years ago, most South FL offenses were nor running high octane spreads, they would mainly take the most atheltic kid on the field, put him at QB & use him as an extra RB for the most part, the ones who could actually throw were the ones who excelled & were able to play at the next level.
The 7v7 circuit plays a big part too, kids are getting year round reps playing in 7v7 tournaments & they're throwing & operating offenses a lot more.
There's a lot of factors to it, resources & proper training are the biggest factors overall, it's not a race thing at all, because if you look around HS & College football there's Black QB's exceeding at high levels who have great arm talent & they all had the same coaching/teaching as some upper middle class kids would have.
Look no further than our current QB D'Eriq King, he's a coaches son & has had private QB coaching his whole life & in HS he broke all of Kyler Murray's state records.