I’d be sparing with your use of the word ‘dumb’ if I were you. That’s not what she said.
Explanation:
The original poster said, why should her boyfriend, a teacher, be forced to teach pupils to understand financial products, because why couldn’t parents teach them those things themselves. No skin in the game there, then. You’ll remember that the subject of the thread is, paraphrased, if kids are going to be taught maths in some form until the age of 18, should some of that teaching cover mortgages and pensions?
The poster whose response you called ‘dumb’ pointed out the logical flaw in the original poster’s argument, which is, if your argument is “why should teachers teach X [where X is any topic], the parents should be doing it”, the same rather weak argument could be extended to any other topic, especially those areas of knowledge less frequently used.
Which, as you note, would be to everyone’s detriment, but the second poster is absolutely not saying, “we should only teach basic maths skills that everyone uses regularly”.