It doesn't assume that at all. If the valuation on the deal is below your estimation of the true enterprise value, then this mechanism allows you to participate alongside the others and also buy cheap shares.
If not then you find the money somehow. Rich people don't have piles of money lying around. They just know how to convince others to lend them money at a certain rate, and hope that the profit pays all that off and leaves them a chunk. It's not magic; it's just being relatively trustworthy with risk.
