"I doubt it happened"

The Victorians seem to be ambivalent about the use of "I doubt it ... ." cf "I doubt it," said the Carpenter, and shed a bitter tear. which does mean (i think) that he did not believe there was any real possibility of the sand being swept away.

But Dickens abounds with the usage that you refer to, Bingley, as does Trollope.

I think that it was apparent to the contemporary speakers (and readers) by the context in which the phrase appeared