I don't think he is a moron, just "in our view" silly enough to want to try and attempt it knowing there is probably greater than 1 in 2 chance he will not return. He must obviously either think he has a good chance to do it and be safe, or is so hot on being "The One" to make 400m, this is clouding the risk factor for him. I suspect in his mind this is doable. In our mind not only not achievable, but stupidly risky.
I couldn't even get my mind around a 28 hr run time, let alone carrying all the gas required, or the potential effects of He and N on my body at those depths. I would not be prepared to accept the risk of a 1 in 2 chance (its probably 1 in 1.2 or less) of not coming back, or being so badly bent to be a total cripple. For me the rewards are not great enough compared to the risk.
However in saying that as I said previously, I would go to 100m as I see the risks are much much less, and they are known given the number of safe dives to date to that depth (compared to the few to 300m let alone 400m).
Probably for the majority, its the near total unknown that makes us shudder at the thought of 400m dive more than anything else, based on what we know at 50, 100, 150m diving.