That’s true of any historical document; it doesn’t imply that what Hitler said about Christianity should be rejected. Kershaw’s wording in its context (which you omitted somewhat) is notable; he says that even the table talk monologues should be treated with due caution, suggesting a level of reliability that is superior to other sources.

Another example is this paper by Richard Carrier. (Carrier is an atheist activist, but nonetheless he is also a respected historian and this is a peer-reviewed paper)
A respected historian? I mentioned how he wasn’t so reputable in post #110 and explained why in post #34.