It would be simple for them to say 'this is not a serious offer' no matter the premium that is a good faith way of rejecting it.
I could offer them $150 a share tomorrow, contingent on financing and would get laughed out of the room.
I could offer them $150 a share tomorrow, contingent on financing and would get laughed out of the room.
My point is that the number on the offer is not an objective and singular measure of whether this is a good deal for Twitter. Who is making the offer clearly matters as well to how serious it is.