According to Boycott and Bourquin (and others), several of the Kinixys species do in fact hibernate to one degree or another. The southern populations of belliana, spekii, lobatsiana and probably natalensis all become dormant for up to four months in southern Africa during their winter. Of course, as has been mentioned by others here regarding 'hibernation' of tortoises, that does not mean they are out in the cold exactly either. Most authors when speaking about these tortoises mention that they seek shelter in burrows of other animals or otherwise find a stable environment in which to pass the colder months.
But, the equatorial species like homeana and erosa should be kept warm and very humid.