If this is actually how it works, you guys are doing DLC right.
They are, by distributing alot of the new content via patches.
It means everyone gets Mali and the new combat system (Im sure Im not the one who noticed the 'winner' was taking high casualties in this video, something that never happens right now). Everyone's AI Muslims will behave and play by the rules of the new system too. All this ensures Paradox only has to work on a single 'code base'.
But you have to buy it if you want to try out the new Muslims yourself, and thats a big enough carrot to ensure everyone who would have, will buy it.