I think a "zero day" threat model and terminology is from the point of view of the blue team type of people running the system, not the vendor. From someone ru

I think a "zero day" threat model and terminology is from the point of view of the blue team type of people running the system, not the vendor. From someone running a system, it doesn't matter if the vendor had 0 prior knowledge of the vuln or if they had made 25%, 50% or even 99% progress towards a patch. The point is there is still no patch available for the vulnerability and your only defense strategies are the same as if the vendor hadn't known at all, so it's still a 0day.