Remember Musk wasn't ever threatening a hostile takeover anyway. He knows it won't work. He was making an offer to the board to induce them to bless the sale. And the board responded with the poison pill, essentially as a way of saying "no". This is the way this kind of negotiation works.
And from the opposite perspective, arguments like "the board isn't performing its fiduciary duty by accepting Musk's offer" are likewise silly. The board is elected by the shareholders (the actual bureaucracy and process for this varies between corporations, I know nothing about Twitter). The level of oversight needed to ensure fiduciary fidelity is already there. No court is going to view a board trying to oppose a hostile takeover as a breach of fiduciary duty, that's ridiculous.
Also, can the shareholders then sue the company (successfully? :)) for decreasing the price? ("everything is securities fraud" after all.)
Nope. The poison pill allows everyone but the one who triggered the provision to buy shares at a discount. From Twitter's press release:
>In the event that the rights become exercisable due to the triggering ownership threshold being crossed, each right will entitle its holder (other than the person, entity or group triggering the Rights Plan, whose rights will become void and will not be exercisable) to purchase, at the then-current exercise price, additional shares of common stock having a then-current market value of twice the exercise price of the right.