Yep.
And that week was also the first week of T5 post-nerf. But ofc the dip was because of XMas, it happens in every expansion.
Pre-nerf is way more fun and TBC didn’t die earlier due to Vashj/KT being too hard. Its clear Blizzard only toyed with the idea of a full post-nerf release cuz they don’t want to spend time dealing with Anniversary patches and potential bugs - specially since a lot of the backend stuff is new and China matters more.
It has nothing to do with player retention.