symlinks (or whatever windows calls them)

Windows actually has two types of symlinks:

• Shortcuts: stored as regular files on disk; only function as a symlink from Explorer.
• Actual symlinks: stored as actual symlinks (or NTFS reparse points). Transparent for all apps, but can only be created using mklink.
• There's also junction links apparently, but afaik they're just bindmounts.

moving a symlink can sometimes move all the data too.

Probably, someone managed to create a real symlink in their OneDrive folder, and since OneDrive probably doesn't check for symlinks it blindly copied all the files to the cloud.

Take all this with a grain of salt — I'm not a Microsoft developer, and it's been a while since I last used Windows.