Can we derive parent’s private key using child’s private key?
But not the former. Can anyone explain what he meant in the former?


A child key can refer to two different things: the child private key and the child public key. With just only one of these two things, you cannot determine the private or public keys of any parents or siblings. Just revealing one of those keys does not necessarily reveal any other information.

However, when a child private key AND the parent extended public key (so public key and chaincode) are revealed, we can derive the parent's private key. This requires more than just the child private key and cannot be done with a child public key.