When registering Tag Helpers, it’s the assembly that is required, not the namespace – explained in the docs.
…the second parameter “Microsoft.AspNetCore.Mvc.TagHelpers” specifies the assembly containing the Tag Helpers. Microsoft.AspNetCore.Mvc.TagHelpers is the assembly for the built-in ASP.NET Core Tag Helpers.
So in your case, you can just change this:
@addTagHelper *, X.TagHelpers
To this:
@addTagHelper *, X