Episodes are filmed 6 months to a year ago. Would it be a 0-day at that point? I remember when 0-day actually meant brand new (typically a release on that particular day), but that was 15-20 years ago. Over that time, the definition could have changed.
That's still the definition. But if they filmed an episode with an unknown exploit, it didn't get leaked and nobody else finds it between filming and release, it would be 0-day. Very questionable to do though.
It would be controversial but really interesting if they dropped an actual 0-day during the show as well.