I would guess that even the people with 29/30 composite scores in TBP had relatively high math subscores. It really doesn’t make sense to be on an accelerated track in HS (taking calc 3, diff eq) while also performing poorly on ACT math.
It’s certainly not a rule, but generally speaking students who succeed with higher-level maths score very well on ACT math. It’s definitely not the case that a high score implies success later on, though, since as you said ACT math is not particularly difficult.