As the developer explains, anyone who has written code to develop mod software for the existing version of Skyrim will have to put a significant amount of effort into supporting the anniversary edition.
This has meant that mods that run on the existing version of Skyrim are not compatible with the new anniversary edition.
Mods developer explained this because Bethesda updated existing versions of Skyrim and changed some of the code.
7 t a separate entry to download on Steam, but rather an update to your existing Skyrim Special Edition installation, and it could cause existing Skyrim mods to not work.