Here is the contradiction that I see:
If he is such a great fan of the estate tax, why isn't he allocating 1/3 of his estate to the government? After all, it is his belief that the government can spend the money so much more wisely than a private charity!
I think the obvious counterargument is that the estate tax, to some degree, incentivizes this type of charitable giving. So from his perspective, he gives billions away, while still retaining billions that will be taxed upon his death. That's a win-win. The gubmint gets a nice chunk of change (and that's money already spent, so there's really no point arguing who can spend it better--the gubmint spends whether it collects or not.), a charity gets a big chunk of change, and the money gets spread around, eliminating the threat of a monied class emerging.