Yes, exactly. Video ID is just a base64'ed DES-encrypted primary int64 video key from MySQL. It used to be sequentially incremented until at some point they switched to randomly generated primary keys. Any (ex-) engineer who snapped a copy of the encryption key (it used to sit right in the code for anyone to see) can enumerate all videos from YT until that moment, including unlisted - which are only protected by secrecy of that one key. If the key leaks, then also anyone in the world can. That's what they are afraid of here. Source: worked for YT.