It’s very much true that wealthy parents supplement their kids to keep them challenged and leveling up, so to say. What you’re describing re: AP classes is for high school students. While the parent comment may have been referring to high school, I imagine they meant elementary and middle school level. Students with support (read: challenged to learn at their level and not slowed down to the pace of the average student) are able to take more AP courses because they are ready at a younger age. They take AP Calculus in grade 9 or 10. My son, for instance, is taking algebra 1 as a 6th grader because we started doing math lessons at home for fun the last couple years. In terms of AP science classes, it’s typically hard to take all of them if you’re not doing outside lessons due to the nature of prerequisites. And, back to the point of extra lessons (which only wealthy parents can afford) I’ve had a few 8th graders (learning programming with me starting in grade 7) who scored 5s on the AP computer science A exam. Often students can’t get to that AP level without additional support prior to high school.
The reason parents look for extra lessons is because most schools can’t challenge students because they group too many students of varying intelligence and interest level into the same class. My public school district does 1 on 1 mentors for students, but only if they score higher than ~144 on an IQ test in grade 2 or 3, which is ridiculous, but these students do get that extra challenge and support with no extra cost. Schools need smaller cohorts to best support kids of all levels, and we’ll continue to fail the majority of kids until we reorganize our schools.