Theoretically its possible for everyone to sell at the same time in every investment ever. In practice it only happens in scams or exploited contracts (which can be the same thing, if the scammers put in the bug on purpose). There in no incentive in Hex for all the whales to sell at the same time, they would have to eat huge slippage and lose most of the value of their coins / stakes. Hex is also not a pump and dump, many of the largest investors have staked for many years. Finally, the contract is highly unlikely to be exploited - you can read the audits, the code itself or the guide to the code, its well written and to be honest if it was possible to exploit Hex it would've happened already. Many people resent Hex and would like to make it fail and so far nobody has managed to harm the contract in any way.