You are entirely correct. I'm not disputing that.
In his time at Garfield High School in LA, Escalante did a great job with his calculus class:
Escalante continued to teach at Garfield, but it was not until 1978 that Escalante would instruct his first calculus class. He hoped that it could provide the leverage to improve lower-level math courses. To this end, Escalante recruited fellow teacher Ben Jiménez and taught calculus to five students, two of whom passed the A.P. calculus test. The following year, the class size increased to nine students, seven of whom passed the A.P. calculus test. By 1981, the class had increased to 15 students, 14 of whom passed.Garfield High has an enrollment of over 4,000 students, and is 99% Hispanic. At his peak, Escalante found 73 kids out of that large student body who he could get to pass calculus. At his peak essentially selected the top 10% of the student body, in terms of math aptitude, and put them through a 4-year program of rigorous instruction in Algebra thru Calc.
What he demonstrated was that he could take the top 10% of the Garfield population, and have them achieve.