(I apologize in advance if reopening this thread is out of bounds.)
The report isn’t about using Obsidian as a general default editor. It’s saying that Obsidian fails to handle the files in its own app folder as iOS / Files expects it to.
Files appears to treat all files in an app’s folder as belonging to that app. So when you view an Obsidian vault folder in Files and tap a note (or an image file, or whatever), it tries to open it with Obsidian instead of the usual default app for that file type. Obsidian launches but doesn’t open the file — so you can’t properly open anything in an Obsidian vault from Files. In the context menu you can use Quick Look to preview a file or you can share it to another app (which I believe generally sends a copy). It is possible to open files from other apps via their integration with Files, but you have to start in the app, not in Files.