My understanding of the most real world related, prime philosophical message of the Second Apocalypse is determinism. Men are ruled by a myriad of innate urges, the Legion Within, without ever realizing that their actions do not belong to their free will, but to these urges. The Dûnyain realize this, and even though they can't entirely free themselves, they can abuse it to rule others. To me, Serwa thinking that others were not real was an extension of the Dûnyain philosophy, just put in a unique kind of way because she did not get the full Dûnyain education, and is worldborn.
Why does she think Sorweel is "real", though? Surely, the Nonmen Sorweel merged with didn't make him free of the determinism, as he seemed as ruled by his passions as any man, if not more. I think at that point, Serwa kind of had a crush on Sorweel, because he was suddenly this hardass ancient soul who also happened to save her life and trigger the downfall of the collaborator Nonmen (and Serwa is still worldborn, after all - and it seems like even Kellhus fell in love with Esmenet judging by that line about how losing her will sink his heart into ruin or something like that), but it'll probably get explained better in the next book. I do feel like Bakker is setting things up for a second generation kind of thing, with Sorweel and Serwa, Kelmomas and Kellhus's full blooded Dûnyain grandson, who has none of the Dûnyain anti-emotional conditioning but all of the intellect.