The "source material" in this case IS the author's VN. It doesn't have to be from Type-MOON or some other AAA publishing company to be a source material. And since the sequel's ending, (which is out and complete now) nearly the entire cast is set to be made enemies of some one now anyway. What with the revelation at the end of this one.
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Cue "I don't think it means what you think it means" comment.
"source material" implies that this story is closely based on something that's already been written, and follows at least the general storyline of said work. Think Ready Player One and it's movie adaptation. The book would be the source material. The basic plot and everything is pretty much the same, even though details are different, or missing altogether, but it is pretty much the same exact story (starts the same, follows a similar route, and finally ends the same). This is not the case here.
So hence why the "there is no source material" line. Sure, this is a sequel to something, but the thing that came before is not the source material, as that would imply that this is just a rewrite of Rising Angels: Reborn.