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.