There’s also the thing that I’m sure it’s possible to read the directory and the files in it if you don’t look too closely at what you’re reading - just hold your nose, open the file, read the contents. But we take great care to not follow symlinks, not read or wipe out things like named sockets and pipes, and so on.
Hence, when the OS throws us something that is sort of like a file, sort of like a symlink, maybe neither and acts weird when you try to figure out what it is - we will take the safe route and skip it. Unison and other programs may have a different philosophy and take a different approach. That’s fine.
This is also a scariness around even trying to support syncing the OneDrive files. I assume that if you actually check that box, files will no longer be on disk unless they are in use, and then get automatically offloaded into the cloud. Syncthing will replicate that as a delete to everywhere. If that everywhere is another OneDrive folder, that will look like a regular user-initated delete to OneDrive. Poof, gone for real.
All in all, while it would be nice to support whatever funkiness Windows is throwing at us here, I think it’s probably a Bad Idea™ to sync your OneDrive folder using Syncthing.