The poison pill is essentially a gun to the head of all the other shareholders: you're either with us or against us. Help us fight this hostile takeover or get diluted along with Musk. How will people react to that?
Some will jump at the chance to buy discounted shares. Others will be unable or unwilling to buy more, and sell to avoid dilution. But they'll have to sell to people who aren't currently shareholders, because shareholders have a better option. That'll drive the price down, and also reduce the discounted price for shareholders. Will investors that buy in after the poison pill is triggered also get the discounted price? Will existing shareholders wait to buy discounted shares until after the exodus lowers their price?
There are so many feedback loops and non-obvious motivations here. It's possible that Musk could trigger the pill, stand back and watch the stock crater, then buy it cheap. And if the stock does crater, will he then have to compete with other takeover bids?