Not just "any" 1990 guitar, of course.
I myself recently bought an early 90s guitar of a series that has been discontinued after few units were produced, and yet I think of it as non-vintage or maybe I'd say semi-vintage...
The term vintage is defined by the Merriam-Webster dictionary as:
—used to describe a wine usually of high quality that was produced in a particular year which is identified on the bottle
—used to describe something that is not new but that is valued because of its good condition, attractive design, etc
—used to describe something that has the best qualities or characteristics of the things made or done by a particular person, organization, etc.
So, I think it has less to do with the instrument being an "antique" and more with it being particularly good or rare, though not new. I tend to think more or less like that...