While it might not be obvious and certainly is not optimal, you can hit Return twice on the item that names the module in the lower part of the tree view, which first opens a folder browser opening the folder containing that module, and then selects that folder. That's how you can get out of that folder again. Backspace is tied to doing the same thing as the ".." item in the upper part of the tree view does, and as that part still points to the containing folder, backspace goes one folder up.

r15680 adds a context menu item to the first item in the instrument browser to close it, and backspace now also goes back to the containing folder (both only applies in the lower half of the tree view, where the individual samples and instruments are listed).

There's one more small thing. Maybe this is intended, but when backspacing to the containing folder the cursor position is not restored. Everything is great otherwise. Thanks for fixing keyboard accessibility issues. This is one
of the things that make this project really outstanding.

Right, this should be fixed as of r15682.