In saying that 11:30 AM was in the small hours of the morning, Bertie is using

Betty is using a hyperbole

Explanation:

Hyperbole can be seen as a literary device, where the writer or speaker, tends to exaggerate occurrences so that they look a lot more intense than they are. This is usually intended to achieve a dramatic effect.

For Bertie to say small hours of the morning instead of "early hours of the morning", Bertie intends to draw a great deal of attention to the time, making it feel as if 11:30 am is earlier than it should be, creating the effect which makes 11: 30 the morning to be a very early time of the morning, meanwhile, it is in fact very close to the end of the morning hours.

This exaggerating effect given to how early 11:30 am is, can be seen as the literary device called a hyperbole