Rout
IMHO, the Russian unit can rout away.
Surrendering doesn't come into play. A20.21 say (first, edition, but I don't think this has changed) that a unit will become a prisoner only if "ADJACENT to Known, Good Order, armed enemy Infantry/Cavalry". It goes on to say "If the only adjacent enemy unit is a vehicle, the broken unit must rout away." So the unit must rout away.
Since surrendering is NA, the ordinary rout requirements come into play. The unit can Low Crawl to M5 (not ADJACENT to an enemy, no closer to an enemy), or it can rout thru M5 (risking Interdiction) into M4.
I think the key is: the tank forces the broken unit to rout, but cannot accept a surrender, so the unit routs normally (if possible, which it is in your example).