Hey there everyone, I believe there may be some confusion surrounding this subject and I would like to clarify.
PlayStation users can pre-load the expansion. If you are on any other platform, you will not be able to pre-load the expansion, you will be able to download it and play it, once it launches tomorrow March 10th.
Some users on all platforms are able to access/download the expansion now. However, this is because those users either prepurchased the content at an earlier date or they are using Ubisoft+ (excluding stadia users).
I hope this clears it up and I would like to apologise for any confusion caused. If anyone has any other questions or experiences any other issues regarding this, please do not hesitate to let me know.