It is by most definitions I can find. Why would being a "subscription based business" exclude it from that? Most social media sites are companies and monetization by subscriptions with revenue sharing is far from rare.
The policy states that employees should not communicate online in a manner that’s “unprofessional” or that would “significantly and adversely impact [their] work-related reputation.”
so·cial me·di·a
websites and applications that enable users to create and share content or to participate in social networking.
It absolutely meets the media part of things, and while it’s not a place for just casual “socializing” you’re still communicating with others. It’s not what we’d first think of as social media, but then again people somehow think lemmy isn’t social media.