I think the point is when you're starting off you have to wear many hats, and since HN is a lot about start-ups, you'll see that often here. At a certain size/load I think most people on HN would recognize that you need specific talent sets (and they might not be interchangeable).

They aren't interchangeable, but there is a significant amount of overlap in the position, especially when you begin operating at scale. The amount and type of automation required when you're dealing with thousands of servers demands a breadth of knowledge that spans both domains.

They're synergistic roles, yes, and ideally you would like one with a mix of both [either a dev with sufficient experience in system administration or a system administrator with sufficient experience in development] to tackle the messes that crop up, but I haven't seen anyone claiming they're interchangeable - just that most of the population of HN is some overlap of the two.

This is likely because many of us are part of small startups where the roles are often interchangeable in the early days: the same small engineering team is responsible for both "writing code", backups, infrastructure, and everything else technical.

It's very difficult to have your foot in both worlds and remain current in either. I did for a while, but I'm more firmly planted in the dev world now. So I can say, with some confidence, that they aren't very interchangeable.