Hi Benjamin - Thanks for sharing your research! It's always interesting to see a different perspective, and that's exactly why it's important to test everything on Facebook.
Since the "When your Fans are Online" graph is unique to your brand's fans, it's not necessarily true for every Page that posts will be competing with too many other posts - your fan's friends aren't necessarily online at that same time creating stories. and other brands have a different mix of fans - so they aren't likely to be posting at the exact same time, either. However, I can see how friends would have really similar schedules, so posting when they're all online would definitely be a problem. Every brand should run a test for themselves to see what the best time to post is, but when your fans are online is an excellent starting point.