What was he at other levels?
The presence of "at the board level" is distancing language. If he was personally fully behind the deal, he would be likely not to include that phrase and to add that information in a different structure.
My take is he's uncomfortable with the deal and was likely not fully behind it.
Bill's role is at the board level these days. He's not actively involved in the day-to-day running of the company.
But a decision of this magnitude is, without a doubt, a board-level decision. And there, at the very highest level possible, Bill was a strong proponent, he says.
And somehow you read this as him not being fully behind it?
If you take everyone, particularly people with great power, at the surface level of their language, you are missing most of the meaning. Indirect and qualified statements are usually that way for a reason. Read up on this - I'm sure you'll be fascinated.
Agreed it's distancing language but my guess is that he doesn't want to undercut Ballmer's authority.