Are file permissions on macOS cryptographically enforced?
The file permissions are only enforced by the operating system. If you boot from another OS (where you have root user access), you can read anything. 

Of course, if the disk is encrypted (research Full Disk Encryption), then you would need the key to decrypt it before you could mount it. On a Mac, the disk encryption key is typically protected by the users password and decryption is handled during login.