You have my quote (complete with details) - are you saying the author made it up? - Jim Carroll

You quote an author writing about an historic event.

I have supplied the notes taken at the meeting by one of the Irish Delegation.

You have been unable to point out anywhere where this supposed three day ultimatum came into any conversation held at Downing Street on the afternoon or evening of the 5th December, 1921. So yes the author made it up.