Am I right in assuming that not enabling PIN; is a self decryption device?
Yes you're right in that without the PIN enabled decryption will occur automatically without any user input/authentication required.
However this does not equal having no hard disk encryption at all. The data on the disk is still encrypted which means that if an attacker physically removes the disk to read it or boots onto another OS (from a USB), they will not be able to read the data on the disk. This could deter casual attackers with limited technical abilities.
More sophisticated attackers may be able to recover the encryption keys by other methods and thus will be able to decrypt the disk. This may be accomplished by first booting the operating system and then extracting the keys from memory. Alternatively the may attempt to extract the keys directly from the TPM chip. Or they might attempt to exploit unpatched vulnerabilities in the OS or services that launch on startup (like RDP or SMB) after booting to achieve Remote Code Execution on the machine. F-Secure has also demonstrated that it is possible to sniff the key off the SPI bus.