but it’s not comparable at all unless the math you’re doing is simple arithmetic.

That's what they were studying on the math side

For the first experiment with 354 participants, one group was tasked with solving 15 basic fraction problems

Also

If you’re doing calculus, the skill you’re working on isn’t arithmetic, so the calculator isn’t much of a factor.

Calculators would be a big factor in solving calculus problems. Haven't done calculus in a while but if I recall correctly you are breaking down or rearranging the problem into smaller steps that are more basic arithmetic. One part of a calculus problem could be finding out what the square root of 7 is , if you have a calculator to do that you're more likely to solve the problem more accurately and to not give up on the problem, which were the two metrics this study was looking at.