I think your reading is being uncharitable; 'substantial' doesn't refer to any particular amount, just that the amounts are meaningful, which will be context-specific. (I also like the term because it isn't deceptively conflated with a technical meaning, like 'significant' is - significant could mean either substantial/import/large or meets an arbitrary p-value).
The way I see it, he's stating that basically 0% of students are trained the way he would like them (surely true), that 0% is not optimal (also surely true but I'm not sure what the optimal percentage is), and that right now, we don't even have good ideas of what is possible for the various qualities of student because the research simply hasn't been done aside from some suggestive anecdotes (true).
That hardly seems like a substantial fraction to me.
There's something like 15 million high school students in the USA; that's 75,000 students. How many of them are in gifted summer programs?
Overall, this manifesto was quite a read. Reading it gave me a weird sort of feeling - I didn't learn very much that was new, but it's a little shocking to see someone cover in a single place so many topics of interest to me in a competent manner and with an attitude & writing style similar to my own.
I think your reading is being uncharitable
I hoped so too, but didn't have time to read the whole work (which might have settled my concerns).
How many of them are in gifted summer programs?
The range 5,000 to 25,000 seems like a decent 50% interval to me. So yeah, it could be higher. The real issue, in my mind, is not summer but the rest of the year; it seems odd to me that we don't have boarding schools for students that are 1 in 10,000, where they can learn about a high school course a week, or do things like this.
The soon-to-be-resigning Dominic Cummings, advisor to the Education Secretary of the Coalition government, has released a 250-page manifesto describing the problems of the British educational establishment ("the blob" in Whitehall parlance) and offering solutions. I post this here because both his analysis and recommendations are likely to be interesting to LW, in particular an increased emphasis on STEM, broader knowledge of the limits of human reasoning and how they relate to managing complex systems, an appreciation for "agenty"-ness in organizational leadership, whole-brain emulation, intelligence enhancement, recursive self-improving AGI, analysis of human interactions on a firm evolutionary-psychological basis, and a rejection of fashionable pseudoscientific theories of psychology and society. Relevant extracts: