I think the explanation is simply that she notices something inside the person she is talking with. She is not aware of having a tadpole so she does not know what it could be and calls it a shadow.
That is the point. Consider that
the priestess' attitude towards your character only changes after you attempt the mind meld. Before you are part of the flock, after she tries to dispose of you, claiming to do the absolute's will. At this point it's clear that the absolute doesn't have free access to your mind like with the true souls. If you don't use tadpole powers, it won't even have access to your dreams.
Shar is powerful enough to weave a shadow shield strong enough to shield you even from a psionic pseudodeity.
