Microsoft would always be the first to know of the defenders–any other defender would just tell them. It then makes sense to count from there, rather than have multiple counts for each level of people learning of the vulnerability.

I think your argument makes sense when the day counts are close to each other. There really isn't any difference between a 55-day and a 57-day, nor does it make sense to account for some sysadmin who took a vacation day.

I still think that your usage of "0-day" breaks down precisely in the case we're in currently, where the vulnerability has been exploited in the wild and Microsoft has known about it for some time, but there is not a patch available, and the general public (everyone who has to defend against the exploit) found out about it today.