It turns out File Explorer (always) uses tab and shift-tab as synonyms for down and up in its own path completion UI, at least in Windows 7 and Windows 10 that I have tested on.
Accordingly, we have added the same in Opus for the next update.
I don't think the option in the article you linked is relevant. It doesn't change how the tab key is treated or turn path completion on; it makes it so the first match is automatically typed for you as soon as you type a prefix of it.
FYI we are very familiar with Linux and several other OS, including Windows itself, where command prompts / text-mode shells / CLIs cycle through path completion matches via the tab character. This is not esoteric knowledge. 
Not that it has anything to do with this thread (FAQ: Ask one question per thread), but favorites in the folder tree cannot be expanded at all, by design. The favorites branch of the tree is intended to be a fixed list of shortcuts that stays at the top of the tree, so the list is always in the same place and structure for you to pick a favorite from. If it expanded, it'd become a mess and no more use than the main parts of the tree. As you navigate to another folder from a favorite (e.g. the child of a favorite), the tree jumps to the real location of that folder. So the spring-loaded option doesn't apply there as those nodes in the tree don't expand. I guess they could just when doing drag & drop, but it's not something anyone else has asked for so far. (If you want to complain about this further, I suggest your Spring-Loaded Folders thread instead of here. That way, if someone else wants the same thing, they might find the discussion and add their vote.)