Can you just not read and enjoy, A man is in the warp and is not dead so, the warp has no rules the warp is beyond the comprehension of man, you expect something to die in the warp, but the very nature of the warp is that you can not expect, so don't.
I certainly wouldn't expect the Warp to fuck itself over, if you'll pardon my language, for the sake of a single human being. It's been ten thousand years since teh Emprah was placed on the Golden Throne; why haven't there been more examples like Draigo who go around killing everything in the Warp? And if there are, then that leads me to my original question: how hasn't the Warp been conquered by these mighty examples of humanity?
Both extremes are bad; one extreme is improbable given previous situations in which mortals perished in the warp, and the other extreme contradicts established fluff in which the Dark Gods are supreme in the Warp. The only way this could be made probable is if it is made accessible - something like "sometimes humans survive in the Warp, but it is an incredibly rare occurence." Ward has not done this.
A common phrase concerning the willing suspension of disbelief is that "You can ask an audience to believe the impossible, but not the improbable." We all know that the elements of the 40k setting are impossible outside of fiction, but even fiction has limits. We can hear about an organization of super-human beings hunting daemons in a dystopian future and take it in stride; that cannot be said about one who gets lost in the warp and is able to survive because he's so badass and kicks so much daemon hind-quarters that daemons are scared of him and his manly manliness. Such is highly improbable even in a setting with so many impossible elements because we expect him to fail (even for a man of Draigo's caliber) yet he succeeds for an unstated reason without changing the odds. If there was some stated reason, be it a "known known" or a "known unknown", then it would be more believable. That is to say, if the reason(s) for Draigo's survival was listed in detail or if it was stated to be unknown at the time, then his fluff would be much more believable. As a reader, I can't just sit back and take that Draigo can survive in the warp; I have to ask why.
tl;dr Some readers can't enjoy the fluff because they don't believe it
That's just it. SOMETHING is going on. Humans can't survive unaided in the Warp. With Draigo, whatever is making him able to survive inside the Warp could be bending the ruled of 'Warp Reality', causing things to behave differently.
It wouldn't make sense for him to just be 'lucky' and survive. SOMETHING is making sure he survives AND behaves like a Demon, always being pulled back to the warp.
I guess we'll just have to wait until April to find out. I don't really have a rebuttal at the moment.