To the contrary, if you read into the full context of this thread, the underlying contention isn't that we're incompetent at decimal arithmetic, but that the rest of the world (and apparently some Americans too) believes our common fractional arithmetic is too much of a mental burden, and I don't blame those individuals either.
Indeed decimal arithmetic is trivial to most grown adults, but that wasn't the point; the example was merely to highlight how just slightly changing a few numbers in the same sequence of operations serves as an effective counterpoint to the grandparent's assertion that "even children can do in their head reliably".
Americans are taught and handle metric units in compulsory school just fine, but most of us also practice imperial units on a daily basis as well. We're just not culturally predisposed to complain when everyone else conveys measure in the SI mks/cgs framework.