Probably a small regional plane. Embraer E-175 maybe? Those flights are usually an hour or so.

I've had nicer Economy seats before. I find it hard to believe that's anything close to First Class.

Outside of America, first class isn’t a relative term, but a fairly absolute one. If a flight doesn’t have very luxurious seats, it might not have a first class at all, and will instead have business class. If it lacks even the degree of luxury necessary to call its seats business class, it might only have premium economy, which is a term for seats which are entirely normal seats, but with slightly more legroom. You’d never get first class that is remotely possible to confuse for premium economy, because first class means something specific, it’s not relative to what else that particular plane is fitted for.

That's domestic first class. Maybe on a smaller plane like an e175, but it's first.

Literally 2 hours before your “edit to add”, an explanation was already provided.

2 hours after that explanation I was still receiving NEW posts claiming it wasn't First Class. People weren't reading the later posts and, like you, claiming it wasn't first class anyway. So one edit with a picture addressed both things.

From the diagram, this might actually be business class,

I don't get to decide what to call it. Its not my airline. Delta calls it first class.

But still, this would never be “first class” on domestic flights here in Australia,

I'm glad to learn something about my friends in Australia. However, this is an airline in the USA and this is what they call it.

or on any of the very many international flights I’ve been on during my time growing up as an expat.

Nor any international flights I've been on, but as I said in a later post, this is a regional jet. In the states smaller regional jets like CRJ and Embraer move passengers from smaller cities to larger hubs for cross country flights or international.