Guitar makes me happy. Feeling stimulated and feeling like I’m progressing makes me happy. I can be productive in guitar. Where’s the conflation in this?

You should read the sentence immediately following the one you quoted. That comment didn't mean what you think it means.

Yeah, agreed. I don't think I know any person that's written their will, for example.

Maximizing productivity in the context of the parent clearly meant "doing the most of the things you find important". That could be writing a will. That could be smoking pot. It depends on the individual's preferences.

I get that misunderstanding the parent comment is an easy excuse to signal your disdain for American cultural norms but it's really not necessary in this context.