I have a degree in math and I strongly disagree with this comment. I was always terrible at memorizing things, and I still regularly reach for Google to double-check formulas and theorems I should know well. In practice, I think a lot of math people end up memorizing things the same way programmers memorize the syntax and tools of their preferred languages; it's not necessarily something you explicitly set out to memorize, but you use them so often that you end up internalizing them anyway.