This episode is a "sorta"-spoof of the 1984 film Red Dawn, which is referenced alot in the episode, both indirectly and directly. It's also an episode of South Park (so no, it doesn't get cocky but it sure gets in your face).
I don't understand the message of this episode, maybe it's to prove that even though old people drive like crap that they still have rights too? And maybe that AARP is like the former soviet union that is going to take over our country by taking our townsfolk hostage? Who knows? All I know is, I had buttload of fun.