I've just reread it again and this is the story of a characte rwho is pulled into the realm of chaos where he can do no lasting damage and the forces of Chaos cannot vanquish him either- its an uncomfortable stalemate- and occassionally he is drawn back into the mortal realm. Its important to note that it never says daemons are afraid of him- they just don't seek him as they can't do anything to him and nothing he does matters anyway.
As Grand Master of the Grey Knights, he's the perfect allegory of futile resistance that perfectly encapsulates the chapter (which is the same idea as the allegory that is Calgar just with their respecive backgrounds)
To be honest I find it a strange hypocrisy when people find Imperial characters doing well unrealistic, but its ok when abaddon manages to charge a blood angel devastator position and survive unhurt, or Gazgkhull Thraka never loses in single combat, or Orks find themselves perpetually reborn on a chaos planet .
none of these things show any more internal consistency than any others- so its just wierd judgements that I have yet to see sensibly reasoned.40k is fantasy in space and has been ever since I started playing (during 2nd ed incidentally) with its larger than life heroes dominating proceedings