There's no incentive not to sell (by the contrary), meaning there's no long term ownership, which means that the system just temporarily avoids the financial incentives to pump-and-dump, doesn't remove them.
If the Founders who get the 10% Founders Reward sell their Zcash (or some of it) before the four years of the Founders Reward has run its course, then that is not pump-and-dump. In fact, that is the opposite of pump-and-dump. That means that the market price of Zcash will be set by a multitude of buyers and sellers, none of whom have a privileged position, and it means that the Founders will have an incentive for the price to go up after they sell. That's the opposite of pump-and-dump.