They compare students who were vs were not accelerated to a higher grade level but does not include option 1) above, which is not a normal class composed of average students of the same age but a class tailored for gifted ones.
Option 1 doesn't seem generally feasible to me. If you are targeting the 1 in 10,000 kids in the article, and want say 10 grades worth of them, with maybe 10 kids per class, then your looking for a population of 100 kids drawn from a population of 1,000,000 kids. Good luck finding a population of 1M kids geographically close enough to be served by one school. Even if you're targeting the 1 in 1,000 kids, the geography is a super hard problem.
They compare students who were vs were not accelerated to a higher grade level but does not include option 1) above, which is not a normal class composed of average students of the same age but a class tailored for gifted ones.