He's saying half of the wealth in the world is just value being stored, and is spread amongst various assets instead of being held in money (because the money the world uses is broken and isn't a store a value). But since Bitcoin is both the best money and best asset (and best store of value), over time, the wealth being held in all these various stores (art, real estate, collectibles, etc.) will move out of its current vehicle and into Bitcoin. Because Bitcoin is the best, and since we're talking about Saylor: there is no second best.

But that's kind of an assumption he is making here, when he says half of the assets could be replaced by Bitcoin.

It is, but it's not a baseless assumption. The money will flow to the best thing, naturally, in time. Plus, the $900T in the world is not static, it'll grow and grow, forever, in fiat terms