I found a player in the recent MCC event who can freely change their own textures

When I was playing on the 15th anniversary celebration server jointly hosted by MCC and the official, I noticed that a player with ID "Bread114514507" seemed to be able to easily replace his skin with some giant image textures in the server lobby?
I followed him around the hall for a while, and he seemed to have noticed me too. When I approached him, he replaced his skin with a transparent texture (only displaying his name on his head), which was really interesting. I really want to know how he did it