I disagree with that. It's far easier to do the math in your head than whip out a calculator and hope you get the right answer more than once in a row.
My kids have beat out public school kids in doing calculations hands down. They do it in their heads in seconds, while the public school kids are still entering it in their calculators and trying to figure out which of the different answers they got is correct.
Of course being able to calculate in one’s head is superior to relying on calculators.
But memorizing the easy cases of one-by-two digit multiplication and the easiest cases of two-by-two digit multiplication is to multiplying several digit numbers mentally as “sight words” are to phonics.