Well that doesn't explain how you can tell with absolute 100% certainty that it is in fact from mt.gox but I would agree they are probably the only ones with this much coin, so it must be them.
You seem to lack some understanding about the very bitcoin fundamentals.
if davout recognized (as his own?) one of the addresses that sent money to MtGox then he could be pretty sure.
yes, finally some common sense
the question is why is the owner moving them to a single address now
good question, from a maintenance point of view it doesn't really make sense, well it could, but then it would probably have been advertised in advance.
from a privacy point of view it's absolutely nonsensical, now everyone knows how much BTCs are in the bank, and everyone knows that hacking mtgox has a big bounty attached (well not that anyone would have doubted it in the first place, but now there's an actual figure)
if thats the full mtgox balance being moved it also means you can pretty much see withdrawals in real time for the next couple of days, that's a nice information leak.
i'd be curious to know the actual answer