1. "Any Wikipedian worth his salt" will look at a source before dismissing it. You - by your own admission - called it "nonsense" without bothering to check where and when and by whom it was published (and it all says on the front page had you bothered to Google it, plus it was referenced). I call that sloppy at the very least.
2. We have a policy for dealing with contradicting reliable sources - it's called WP:DUE. It says that "viewpoints that have been published by reliable sources" are to be given space in the article "in proportion to the prominence" of their viewpoints. So even if we knew the figures in any source about Bleiburg were off (which we can't possibly know anyway) we are not supposed to automatically reject the source in question if it represents a significant viewpoint (in this particular case the differing histories told by historians all over the region do represent "significant viewpoints"). In fact, we are actually supposed to include it and we are actually obliged to do so per WP policy.  (''talk)