I can confirm, NAS is not required. Reopened the bug.

Just started using this.. drag and drop

It appears when dragging a folder (windows folder on a NAS) to Music in MM5, it reads to the library the folder above that that was dragged in the directory structure
ie
Structure is //Music/B/Bob Dylan/Rough and Rowdy Ways 2020
Dragging the folder Bob Dylan it reads all the B folders/directories to the library!?

thanks ill check it out

You can since 2261 by dropping on a root Collection node: https://www.ventismedia.com/mantis/view.php?id=16821

In MM4 you were able to drag and drop a single or multiple folders (on a NAS) and update the database/library with the added (dragged) folders, I've been checking and I don't think or see this functionality is in MM5.
The only way I have been able to update the MM5 database/library is to go: File/Add/Rescan files to the Library (which takes 20 minutes plus to rescan the "whole library", with 100K files).

Did I miss a technique to add a single folder or folders with multiple subfolders to the Library?

I normally review and correct my new music folder additions (structured: Alphabetic Letter/Artist/Album, then add the cleaned corrected/new files in folders to the library for Mediamonkey processing ie Volume/tags etc
or
Add folders with sub folders (Alphabetic Letter/Artist/Album).