RemoveDirectoryTree(some_dir) works, if it returns FALSE to you so
- either your Ensemble instance has no rights to delete files and directories
- or you feed RemoveDirrectoryTree() with a filename or with a  nonexistent directory

Try the following:

write FileUnZip,! // this should be an existing DIRECTORY
write ##class(%File).Attribute(FileUnZip),! // this should be 16 (on Windows)
write ##class(%File).RemoveDirectoryTree(FileUnZip),! // 1=removed, 0=not-removed

In some way, I wonder about your question. This is a very specific question and a correct and accurate answer can be given by Intersystems only. If you take a look on the articles here, the most of them are about other lanuages, new technologies etc. but none of Cache/IRIS internals. If you look on the questions, very often they start with "I'm new to ISC technologies, need some help". Do you really expect an answer from an newbe? Or you really expect an answer from a Kubernetes, Phyton, Angular etc. developer? On the other hand, you are an Intersystems employee, so why don't ask somebody from the developer department? What about contacting WRC? I would be ashamed to ask such a question here.

It seems, you rotate to the left instead to the right...