That’s not been my experience and I was involved for some time coaching high ability and gifted middle school math kids (yes, a large number of whom took the SAT 2 and some of whom had even taken AP Calculus exams).
The SAT 2 is just a test of recall of simple algorithms, requiring no reasoning ability. It’s also not sensitive to ability at the top, as you can get 4-6 questions wrong (out of 50) in a typical administration and still get an 800. AP Calculus exam is too easy because the FRQs only make up 50% of the score, and the multiple choice - again - is simple recall and application of algorithms. The threshold for a 5 on any AP Calculus exam is pretty low - typically in the 55-65% range.
In my experience, it was not uncommon to see, say, a 750 math score on the SAT, but an 800 and a 5 on the AP Calculus exam for the same kid. Actual reasoning ability - as shown by much tougher math exams at a competition level, requiring more abstract or at least multiple step approaches - seemed anecdotally to correlate better with SAT scores than SAT 2 or AP Scores (limited sample, though, especially with regard to AP Scores).
I agree that it is basically equivalent to elementary school math, in terms of scope. But then why should it be that only 5% or so of high school kids heading to college score higher than 700?