That ending is NOT considered cannon. Even the creator of the manga HATES that ending. He has being saying for years that everyone should ignore that shit. He basically denies the existence of that shit. He NEVER intented the guys to die. They NEVER died. In the manga they live happily ever after. Since the anime is just an "adaptation" of his true story, then it is not cannon. Period. It is like saying that the Spiderman movies and cartoons are cannon, which is dumb.