It is not open source at this point but may go that way in the future. I'm hesitant primarily because from personal experience when Mirage Online and Odyssey Online became open source, it immediately killed the player communities. That being said it spawned a game developer community (such as this one), but that does little in establishing a vibrant player community. Why play someone else's content when you can create your own? People take comfort in owning things and being in charge, so it's perfectly natural that players will eventually transition into creators.
As that evolves the player communities dwindle leaving few amongst the vast array of options that are all pretty similar. I suspect if I were to open the source, Facebook, Steam, Google Play, and the App Store would become flooded with "simple 2D MMORPGs" with only surface level differences (graphics for example). The source is as easy to work with as Mirage Online was originally, so I'd fully expect this to happen again, but then again getting more people interested in game development has its own set of benefits. What do you think?