[oefrha]

git-subtree is for incorporating an entire repo. To cherry-pick a single directory from another repo you probably need git-filter-branch to split that out into a standalone repo first, followed by git-subtree. Still, this is just two commands so I don’t see why a third party tool is warranted, especially since the third-party tool here seems to require a more complicated process (maybe it achieves more than what I think it achieves, I just don’t see it from the description, especially considering the problem it set out to solve seems to be exactly what I had in mind).

[AlotOfReading]

Filter-branch / subtree doesn't let you move files outside the subdirectory, as far as I'm aware. Happy to be corrected on this point though.

[js2]

There's basically nothing you can't do with filter-branch and the variety of filters it can execute. Moving a subdirectory to the root is trivial with --subdirectory-filter, but even without that, you could do it with --index-filter or --tree-filter.