Why discuss anything outside of one's domain at all then? There are many examples of intelligent cross-disciplinary contributors throughout history and you've probably met some in your life (especially if you're in software engineering, where many enter the field without relevant degrees). How many people spend 5-10 years in a career and then switch? It's more common than you think. And I'd consider 5-10 years of experience enough to make someone an expert (that's 1-2 PhDs worth of experience!).

Many HNers have lots of experience with finance, both as professionals and as affluent people who have received excellent advice from finance professionals, good educations, and private study.

Obviously you shouldn't ONLY listen to uncredentialed randos on the internet when making important life decisions, but randos on the internet can certainly contribute positively to discussion and spark thought.