I won't be too shocked if, Nexus-like it traps the souls of everyone it kills - giving more to Book seeing his nephew on his ship while investigating the anomaly.
I don't think it is, but to be fair, this is the same series which had Stamets's hallucinations turn out to be true visions of the future and had Culber be replicated by mycelial matter despite Culber having never especially touched the mycelial network to begin with.
Why can't Book seeing his nephew everywhere be the result of trauma? Why does it have to be proof of something else going on?