The vulnerability has been present for more than a decade.

There is evidence that some people were aware and exploiting it.

Apple was unaware until right now that it existed, thus is a 'zero day' meaning an exploit that the outside world knows about but they don't.

Meaning unknown to the public/vendor

https://en.wikipedia.org/wiki/Zero-day_vulnerability

Well whatever the zero means, it can't be the number of days that the bug has been present, generally. It should be expected that most zero-days concern a bug with a non-zero previous lifespan.

“Zero day” has meant different things over the years, but for the last couple-ish decades it’s meant “the number of days that the vendor has had to fix them” AKA “newly-known”.