The board is definitely not a multisig, it wouldn't make sense for several reasons. First, with any company, the board generally doesn't have access to bank accounts for the company so there would be no reason to give them such access with bitcoin through a mutlisig setup. It's cumbersome and needlessly bureaucratic, and therefore unnecessary. Also, the board doesn't really have any power to check Saylor due to his super majority voting power, so there's no reason for him to share responsibility with the board. They serve as figureheads only with no real power in the company. You definitely wouldn't give people like that responsibility to control any part of the company's day-to-day operations, which again would be needlessly bureaucratic.
In that case, it could still be multi-sig and he controls super majority of the keys. An example would be a 3-of-7 quorum, and he has all 7, but 2 other highly trusted employees (not necessarily part of the board, but usually are) have one key each, and the other 5 are kept as backups somewhere safe. Why? What if Michael Saylor the individual gets hit by a bus? The company has to survive and it would need access to the coins. Then one other key can be released by his lawyers or something like that.
The way he talks, it would seem unlikely he has the company bitcoins in third party custody.
Otherwise we are looking at the next QuadrigaCX. (The exchange where the owner with sole possession of the keys died.)