I joined this a while ago just because it's my initials. I still don't really know what ZK is though. Is it still relevant with all the other L2/L3 developments?

ZK proofs are the key technology behind the entire Ethereum scaling roadmap 😁

Cool cool, so it’s not just a Polygon thing?

Not at all, I'm pretty sure every L2 is building a zk prover, many already have it built. It's the only known way to get the instant finality you need for proper transaction composition. Optimistic roll-ups can be cheaper to run for now, but will need to transition eventually.