This is almost certainly users who elect to store their BitLocker keys in OneDrive. Don't think Apple wouldn't do the same. If you don't want other people to have access to your keys, don't give your keys to other people.
As of macOS Tahoe, the FileVault key you (optionally) escrow with Apple is stored in the iCloud Keychain, which is cryptographically secured by HSM-backed, rate-limited protections.
You can (and should) watch https://www.youtube.com/watch?v=BLGFriOKz6U&t=1993s for all the details about how iCloud is protected.