Parts of Spark are indeed "open source" -- but not the important parts, which shows how it interacts with Lightning. Anyway, it doesn't matter -- as of right now, only LightSpark is running the back-end, so if you use Primal's new wallet, you should assume that all your transactions, I.P. address -- basically your entire online identity -- goes into LightSpark's database. Not something I would personally be comfortable with.