are you sure that's actually out of supply? It's showing some equipment as damaged, not destroyed. AFAIK if OOS all losses are destroyed.
No the question was - how is this unit in supply? It is totally surrounded by enemy units, there are no towns to draw any supply from, the unit is hemmed in by rivers with no bridges, there are no tracks - much less roads - that lead to anywhere that are not cut-off. Just seems very strange.
Based on the screenshot shown, it's only surrounded on 5 hexes. The 3 you occupy and the 2 adjacent to it. The 6th to the rear is open, and supply can be traced through non-occupied/non-ZOCed hexes, unless you have a solid wall of occupied/ZOCed hexes to block it, which doesn't appear to be the case from any of your zoomed out shots. Based on the screenshots I'd guess its probably getting 1 or 2 supply points/turn; and if over extended rules are turned on will end up in trouble and start withering away after a few turns anyway. IIRC just being overextended dumps equipment into the replacement pool vs the dead pool for out of supply.
Did you ever see history portrayed as an old man ... weighing all things in the balance of reason?
Is not [it] an eternal, imploring maiden, full of fire, with a burning heart and flaming soul, humanly warm and humanly beautiful?