There is no conflict in a "buy" button and having an open source app. Discourse offers paid hosting services if you don't want to run the app on your own server.

The version on their hosted service is the exact same as what you can run on your own server. This is similar to the model of other businesses like Ghost: https://ghost.org/

Since there's no comparison matrix on the Buy It page, and all of the plans link to the same "Full Discourse feature set", I think it's likely that "Buy It" refers to hosting plans, not the software itself.