I think here とする is acting a bit like a hypothesis/assumption. That is one of the behaviors of this structure.
I think I'd translate it "Buddhism is a religion purportedly founded by the Buddha (aka Siddharta Gautama) of India."
Now, I'm confused by the use of を after 釈迦, personally. 釈迦を開祖 seems like a weird construction. Given my (limited) knowledge of Buddhism, shouldn't it be 釈迦が or の? But I admittedly am not a master of とする.
