Obviously he's not literally suggesting that Gates be taxed $100 billion, and his actual wealth tax plan is nowhere near that level. He's pointing out that, minus $100 billion, Gates would still have more than $2 billion. It's a hypothetical taken to an extreme, not a policy suggestion.
That's literally what he's saying though
Again, he has a wealth tax plan that's public, anyone can see for themselves the rates he's suggesting as actual policy. The $100 billion number he's talking about is a point about billionaires in general, and the accumulation of wealth.