Time Machine Problems
You can read any file on the Time Machine volume - just write permission is withheld in the Backups.backupdb folder. This is all covered in https://support.apple.com/en-us/HT201250

Feel free to explore the drive and use it for other storage - the system keeps you from messing in places where the machine needs to control file writes.

I can only assume the "exclusively" means that it might ask to erase and reformat a drive that doesn't meet Time Machine requirements. I've never had issues with GUID/Extended HFS volumes and Time Machine needing to be "exclusive".