Don't need any more proof that transferring BTC from a block that Satoshi controlled. Don't need reams of explanation or blog posts or anything else.
If the goal is to convince people that he is Satoshi, and he really was Satoshi, this would never be his best choice of action. Given that goal, what conceivable reason is there to delay this simple action?
The most probable conclusion is pretty simple. I'll cop to hedging a bit with "probable", but in the event that this claim turns out to be true, it would be fair to ask questions about why on Earth this was considered to be the correct way to prove the claim.