Yeah, it’s definitely broken.
Reported similar here: Pictish Warchief/Guardian temperature resistances wrong (Guardian is just better for same quality)
As you say, despite the response that got the issue is not in line with the temp changes as they mention nothing about making a single set of DLC armour far superior to anything else. And that’s what has happened - the DLC Guardian armour is flat out superior for temperature management in all scenarios, both HEAT and COLD.