What do you think of the claim that the policy will increase inequality of preparation because better-off parents will bypass public education more? It sounds like you might have more light to shed on that.
But to actually answer your question. Better-off parents already bypass public education, even if not completely. If this new program works (not a given), the problem will not be exacerbated, rather more students will be prepared to reach for these "gifted programs" (which in many cases, even if expensive, usually have good fee-waivers as this gives them more credibility).
[1] Obviously just a personal opinion. But in the interactions I have had in this context I feel I have seen a lot of inflated but unsubstantiated sense of competence.
(I wouldn't bet on it, personally, and I don't think the infusion of wokeness is mainly about helping more people to understand math better. Hopefully I'm too cynical. The Common Core math stuff seems like progress, for instance.)
It is cited in the open letter's supporting document. It would take some very serious effort to read through these resources and try to disentangle the biases of the authors, but I guess this meltdown would not have happened if reading social/ed science papers was easy.
Obviously this is just my personal anecdote, but I do claim I have been very invested in exactly this type of education and have seen how well-meaning crazy-smart professors are failing to teach K12 students (in their once-a-year charity event) while patting themselves on the back for a job well done.