Company looking for buyout would be reckless to make that decision based on stock movement 24 hours after approval or at any time for that matter. They would never know if someone else trumps them in buyout, the company may instead partner for royalties, etc.
In mean time, while they wait for stock to drift down, they lose on future revenues.
Buyers value on what patients are there, what is growth rate, what geography, what is competition, what other application for this drug, how much will be prescribed, what price they can collect, what market share they get and bounce that against putting money in treasury bills and make decisions...Using Discounted cash flows, internal rate of return and net present value..
It's not based on where this stock is trading day after approval.