It was bugged to have stupid high durability.

Kind of, it's because whoever did the script for the Winterised T-51b accidentally pointed it to the sim version instead of the wasteland one (which has the same HP as the normal T-51b).

Most weapons and armour in Operation: Anchorage have a sim counterpart and a wasteland counterpart - the sim versions have incredibly high HP so they won't break while you go through the simulation, while the wasteland ones have normal HP and were probably supposed to be part of the reward stash.

Most of the items weren't placed in-game and are therefore only accessible with console commands, which is a shame since many of the weapons in Operation: Anchorage were reskinned to show them in better condition.