Sure, but "I can't do math" is a shorthand for "I can't do math beyond arithmetic, and even then, I often can't do that in my head." That is not equivalent to "I can't read." It is more equivalent to "I can't read Proust without getting bored and confused."

The author in the OP acknowledges this substitution of "I can't" for "I currently struggle with." It's complaint about semantics that misses the point, I think.

Rather than complaining when people say they can't do math, we should find out why they feel that way. I suspect a large part of the reason is that math education is pedantic and boring (memorizing axioms and doing rote calculations, when we should prove them, for example, or apply math in ways that doesn't involve trains leaving stations) and when people do speak their minds about their difficulties with mathematics, they often face snooty responses like this one in the OP, rather than a lighter touch.

There are three other defense mechanisms at play:

1) I already made up my mind based on emotion or tradition or the salesperson was cute or whatever, and I was hoping the math proves my select decision is correct, but its not looking good and/or I've found a easier / better justification so "I can't do math"

2) I'm getting totally financially reamed over this (housing bubble / car lease / rent / mortgage / tuition / credit card) but its less painful to say I can't do math than admit I have awful financial judgment.

3) I suspect the result of this decision is going to be a minefield, and the other party is more traditionally mathematical than my side's ethnic / school major / gender / job title / whatever so I automatically come out ahead regardless of result by not making any decision and defer via "I can't do math" because first I don't have to put in the effort and secondly I can't be blamed.