Michael and Holly share many similarities, such as their social awkwardness, sense of humour, and a history of getting ostracized by other people. Their likeness had previously made all the things that are wrong with Michael transparent to Holly, and also led to them becoming a functional couple.
For the first time, Jim was able to look like the saner of the two co-managers. It didn't take much on Jim's part, as he was only being himself. This is easy, when Michael was entwined in perceived authority, sexism, and jealousy. His own personal insecurities and entitlement have led to him believing that he has the right to not only be Santa Claus, but to also take it away from Phyllis.