This seems quite clear in the rules:
page 5 of rules wrote:
The defeated general loses as many armies as the negative final score (but not more than he commanded) and is retreated the same number of cities.
In a draw, neither side loses armies or has to retreat.
There is no concept of "can" retreat, as if it's a choice.
It's something which happens to the defeated general, whether or not they want to retreat.