I see the being Anonymous the most reasonable one.

Why would he show himself at this point? Are there benefits for him?

I don't think so, keeping it in secret makes people talk about it which should be fun for him I guess. Again, he has nothing to win showing his identity and a lot to lose, at least, If I was him, I would do what I said.
And about his 1 Mio BTC, I think that he has a lot of money without the BTC anyways or he's just not interested of being ultramega rich, either of those 2 would make sense for me.

Anyways everything thats not confirmed leads to imagination.