That doesn't mean that they're wrong. You can take a look at early 20th century literature to see the sorts of mental math humans (especially humans employed as "computers") were capable of.

Even further back, you can take a look at how good human memory was in oral-tradition, preliterate cultures. All of those extremely long poems had to be memorized by bards, skalds, griots, etc.

And math teachers have been correct. So, if anything, that suggests that the parent post is also correct.

Notice how even with such advanced cognitive atrophy, the patient is still able to repeat that worn-out comparison.