Need more information either way, but with what we have now I wouldn't guess that Illias are people summoned from Earth or anything. That magic explanation said aura was the innate power of this world, and mages use power from a demon world sort of deal. The duke's family are supposed to be part Asmodian or something and those seemed to be demonic, so perhaps Illias are another thing not unlike the Asmodians.
You could argue the point that getting summoned from the demon world is still getting isekai'd for the demons, and I wouldn't really disagree, but the term usually implies it's a human from our world going to some other world (also currently assuming that our world isn't the so called "demon world," mostly because there's no reason to assume otherwise yet and we're not exactly swimming in magic so it'd be kind of weird for mages in this setting to draw their power from a presumably magic-less Earth).