I think what he’s saying is that once you open the preview, you can go to the “share” icon in the top right, select that, and then open in whatever program you want. Then, you should be able to save it back to any place you want. It’s not as streamline as most users would want, but it does actually work.
Correct. I was also saying that actually opening the Word app let's say, and using their cloud connections to Dropbox/Onedrive (no it wont help for icloud users) in the Word app "Open" tab is a lot easier and less steps. Just find your file and it opens right in the Word app you are already in. Then it saves right back to the cloud automatically (or autosave) in Word without any extra steps.
Going through Files to edit a Word doc in Dropbox/Onedrive (absent it being in icloud which Word app will not connect to as a service) is a much slower solution.
Files will not be the most convenient way to do everything, it's not a one size fits all solution.