One of the things that annoy me the most with Sketch, is the seemingly strage behavior with symbol folders.

The UI isn't very helpful in this regard and neither does it seem to be properly documented anywhere in a succint way.

So I contact Sketch support hoping to get some insight. The very helpful Alberto Araújo, a customer support employee replied:

Sketch will automatically apply the desired nesting structure for Symbols with "/" in their name.
However, when only a single Symbol is contained within any parent folder, then Sketch will simply show that Symbol's name in full rather than nesting a Symbol within a submenu.

When two or more Symbols are named with the same preceding title, then the expected submenu structure will be shown in the various Symbol-related menus.
To see this in action, I recommend creating a two Symbols, for example "button / landing1" and "button / landing2".

You will then notice that both the "landing1" and "landing2" Symbols will now be found within a "button" submenu.

Finally!
I understand how it works now.