I haven't looked at older SATs, but there's no inconsistency between the hypotheses:
a) High scores on SAT-M are as rare now as formerly;
b) High score on SAT-M is no longer as good an indicator of mathematical talent as it used to be.

You can make it hard to score high by it requiring insight or by it requiring speed; a shift from the former to the latter would allow both a and b to be true.

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