They don’t need to know the edge cases to understand the basics of how DNS works. It is a foundational element of how the internet works and any software dev should have at least some fundamental knowledge of it (unless they don’t do anything that ever touched networking which I imagine is rather rare).
Yeah, but it's not like those comments are making a mistake about how the tech works because they're looking to learn something today. Posting an axe-grinding comment that shows a clear misunderstanding of the technology on a technical forum is an unforced and pretty indefensible error.