The way I read that makes me think that it could go either way. Njal acknowledges that the powers of the Rune Priests could come from the Warp and that they could all be wrong about Fenris and it's world-spirit. But he personally doesn't believe that and genuinely thinks that what he commands and what he has felt other psykers command are different. And it hints that it might just be possible.
Older 40k supplements make mention of living planets existing in the galaxy, and we know that Eldar Exodite Worlds have their own spirits bound to the planet. Maybe Fenris has something similar and the Rune Priests really do gain power from it.
That said as a die-hard Thousand Son/follower of the Ruinous Powers I say they are full of crap.
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