The usage figures I've seen, e.g. versions of Win10 etc., all rely on monitoring what OS someone's using when they visit web sites, and Ed's article says he used figures from a few different services that did just that. So if/when a VM running 7 connected to a monitored site, it was counted, but if like me you rarely use the browser in a VM, it won't be counted.
Ed also reminds us that Microsoft does provide security updates for Win7 to corp. users who are willing to pay extra. And whatever they add to Win7 to allow the updates [for those who paid whatever fees] has been hacked, so some percentage of those regular users [not corp] are also protected.