That just ... seems weird, right? The show relentlessly fits pop-culture references into everything -- and we're talking deep cuts like My Dinner With Andre and the Traveling Wilburys -- but, somehow, over six seasons, no one ever said anything about Buffy the Vampire Slayer. Even when, again, they were literally students literally fighting the supernatural.
You know what the show almost never mentions, though? Current events. If Buffy was a TV show in Community's universe, Abed surely would have said something about it. But if all the crazy that went down in Sunnydale was real-life to them, they'd almost certainly ignore it outright -- which, y'know, they do.
And, yes, OK, Troy does mention that Firefly exists, and Officer Cackowski, the recurring cop character, mentions Joss Whedon by name in reference to the Avengers movies ... but those clearly aren't the same Firefly and Joss Whedon that exist in our world. All the proof you need is Nathan Fillion.
Chang mentions Nathan Fillion -- star of our world's Firefly, and guest star on Buffy the Vampire Slayer -- by name in season five's "Geothermal Escapism." In the very next episode, Nathan Fillion plays a character who is, notably, not Nathan Fillion. Nobody comments on the similarities, or questions why Nathan Fillion is a janitor at Greendale. Therefore the Nathan Fillion that exists in Community's reality is not the same Nathan Fillion that exists in our reality.