That is how companies get in trouble; even denying a rumor spreads it further, and can have an effect on the stock price by tanking it back down. There are no winners in this game; the investors who bought on the basis of the rumor then have reason to sue as well. Furthermore, a denial is going to be met with ‘alternative readings’ undoubtedly, and now you have a PR fiasco. Instead, this story will blow over in a couple days and everyone will move on with their lives.
Also, imagine if PIF did float a soft offer, or even an LOI. Firstly, they come with NDAs, so any kind of response (positive or negative) may have been illegal, even if they declined the offer.
The safest option is simply to let the rumor die.