I could be missing something.  I see all instance methods in the Utils module.  From the Module docs, "module methods may be called without creating an encapsulating object, while instance methods may not".  There are a few ways to go.  
1) leave it as is and call all ActsAsTaggableOn::Utils through ActsAsTaggableOn::Tag
2) convert all the methods to self methods and call ActsAsTaggableOn::Utils directly
3) add module_function to the top of the module making all methods both instance and module functions.

I am happy to do any of the above.  Let me know which works for you.