The system described in the devblog sounds about as realistic as we can expect. Pre-modern logistics relied little on supply lines. There was less reason for them to have them, and it was less feasible.
In modern warfare, supply is mostly huge amounts of ammunition and fuel, which have to be transported from factories/depots (unless captured), thus supply lines. In pre-modern warfare, most of the supplies were water, food and firewood, which are better to forage/plunder nearby. This will hopefully be well represented by the system described in the devblog. The defending army could reduce the efficiency of the attacker by attacking his foraging/plundering parties, or at least have forces nearby to threaten them, thus forcing them to stay closer together and cover less ground. This will hopefully be a feasible defense strategy in Bannerlord. When an area was exhausted or too well defended, the attacking army would typically move to another area (for example home, ending the campaign).
If well prepared, having carts and travelling along roads, the army could bring supplies for a while, maybe even a month or two. The Roman army arguably was capable of organizing supply lines as an addition to foraging/plundering. However, the main part of these would be by ships, which are unfortunately not planned to be in the game. Long-distance transport over land was not very efficient before railroads and motor vehicles.
There is not so much written about pre-modern logistics, but Supplying War by Martin van Creveld is a good place to start. Two other interesting works are Logistics of the Roman army at war by Jonathan Roth and Warfare in Medieval Europe (chapter on logistics) by Bernard & David Bachrach.