If this is a normal Silicon Valley deal, until Twitter relists or sells the valuation at vest of the already granted RSUs will be $54.20. So from an employee perspective the valuation will stay high.
I would be really surprised if this remains privately-held for very long. It's a LOT of capital (and a lot of it borrowed) so my guess would be that he has an eye on Zuckerberging it (issuing himself a class shares with all the votes, but marketing another class of shares without them).