Parkour Parkour the Office
This doesn't mean everyone knows about it, but that it shouldn't ever be surprising someone is aware of a general knowledge topic without special training or study: it's the kind of thing we just pick up through passive exposure to our cultural environment.
Parkour has been in the public eye a lot longer than The Office, and in much more visible arenas. As the Wiki link you provide points out, parkour is more than 25 years old and has enjoyed a notable media presence for at least 15 years. The opportunities for Harry to pick up on it are vastly broader than a single cult TV show. The "Popular Culture" entry in that Wiki article mentions that parkour is showcased in a Bond film and the Bourne franchise, which are major "common culture" topics off the Internet as well as on (and the kind of thing Harry could -and probably would- see in a drive-in movie).
Thus parkour is nigh ubiquitous: you can read about parkour in newspaper reviews of films (it's likely Harry would read reviews of films he thinks he'd like, even if he can't see them until they're released in drive-in theatres) and in articles about its use in military training.
Such a wide range of exposure in extremely varied contexts means parkour is liable to come up in casual conversation with just about anybody, regardless of their interests. All it takes is someone in a coffee shop striking up a conversation about movies with Harry, or a chat about martial training with Kincaid. It's definitely the kind of thing the Alphas would've mentioned during a Game Night; that time frame is right around the time movies were first picking up on parkour.
As for the supposed Office reference-well, setting aside the fact that Harry does have a tendency to shout random stuff during high-stress moments, The Office is not a purely Internet phenomenon and that episode is five years old. Everything I've said above about parkour being general knowledge also applies to The Office, a show with noticeable real-world cultural impact. The episode in question doesn't seem to fall within the "out of touch" part of Harry's timeline, so that's not relevant at all. Maybe Harry's a Reliant K. fan, or a Chicago newspaper ran a Lifestyle section article about an extremely popular TV show some time in the five years since that episode aired.
Harry's possible points of contact for both parkour and The Office aren't anywhere near so narrow as the question assumes-in fact, his exposure to general knowledge about popular culture is hardly impaired at all despite it being hard for him to see some of the primary sources.