It's just a slight variation on the totally unhelpful "Why don't you fix the bug yourself?" response that's sometimes given to people reporting a problem with an open source project.
They're useless questions because the answer is pretty much irrelevant.
Maybe he doesn't know how. Maybe he isn't qualified. Maybe he doesn't have the time. Maybe he doesn't have the money to fund somebody else doing it. Maybe he just doesn't want to.
It doesn't matter why he might not do it himself. But that in no way means he shouldn't publicly express such interest in such functionality, in case there is somebody else who is willing or interested in adding it.
Well, while we're being pedantic, publicly expressing an "interest in such functionality" deep in some HN thread is roughly as productive as asking him directly why he doesn't scratch his own itch.
This whole notion that saying "patches welcome" is insulting/dismissive/pointless is completely missing the point. Open Source mostly runs on people doing, not pontificating. It may be "unprofessional" or "rude" in other contexts, but it's a fundamental part of what makes Open Source work. I suspect you've never actually spent any considerable time working on such a project, otherwise you'd most likely have seen things from the other perspective and realized this already.
