And what is a Linux distro, if not an OS?

TBF I went in expecting a novel OS.

Calling a custom Linux build a brand-new OS is like a car tuner claiming they built a new car. You didn't build the engine or the chassis; you just tuned it and gave it a custom paint job. It's exactly like a hairdresser - they aren't creating a new human being, they're just cutting, coloring, and styling an existing one. It's a Linux distribution, i.e., a "distro," not a new OS!