Honestly, what the hell? I haven't noticed any alt-fire invulnerability bug, and I bayonet like crazy.
The only time I can pull off something like that is when the submachinegunner misses.
Maybe you're not as good as you think you are.
Edit for thread content: I don't think the bayo dive is intentional, but neither is it a horrible exploit. Besides, it's not like it is out of the realm of possibility for actually happening in real life. It's just diving to the ground when you're holding a stick with a sharp thing at the end. Perfectly possible.
edit number two: I really hope people don't start saying they're good because of that stat tracker. From what I saw in roughly five seconds of looking, it doesn't have anything whatsoever to do with real skill. Just points. Kill/death ratio doesn't matter in the least, it would appear, and that is more a sign of skill than anything else.