Words are used to communicate, and language is fluid and changes over time. Clearly, zero-day is being used and understood by many to mean simply "unpatched", and so that is a reasonable definition. If ever you're arguing that a significant proportion of people are using language incorrectly, you're probably on the wrong side of history.
Don’t you just love it when people pull out their dusty tomes to prove to you that you’re wrong? It’s so pedantic yet also incredibly ignorant of how dynamic languages are.
I got yelled at once for using the word “cheap” to mean “inexpensive” once and wish you had been there with me.
