It's a term of art. In print media, the connotation is "vulnerabilities embedded into shipping software", as opposed to things like misconfigurations.

I think it's a fairly common trope in communication to explain in simple terms any language that the wider part of an audience doesn't understand.

I though zero-day meant actively being exploited in the wild before a patch is available?

Zero day means that there is zero days between a patch being available and the vulnerability being disclosed (as opposed to the patch being available before disclosure).

Discovering a zero day implies that there is no patch, but the term is talking about how long the vendor has known about the vulnerability.

Yes. As a security researcher this always annoys me.