Sense 2 was revived in the 20th century after a couple of centuries of disuse. Although it is a sense with good literary precedent—among others Shakespeare used it—many people think it is a result of confusion with incredible, which is still the usual word in this sense.
Examples of INCREDULOUS
She listened to his explanation with an incredulous smile.
He was incredulous at the news.
Many people were incredulous that such a small fire could have caused so much damage.
“Afraid not.” I made an expression to show that I was as incredulous about this as he was. —Bill Bryson, I'm a Stranger Here Myself, 1999
A tweed-encased fogey, he's allergic to technology, persnickety about language, and incredulous that anyone could object to his incessant smoking. —John Powers, Vogue, March 1998
He was greeted with incredulous laughter. —Robert M. Hutchins, Center Magazine,, September 1968
… no obstacle, no incredulous or unsafe circumstance … —William Shakespeare, Twelfth Night, 1602