The transaction nonce is what is used to create the signature of the transaction or the r,s. It's private as it involves the private key to sign transactions
I'm kinda clueless here, but if it's private, it's not on the blockchain, right?
Ah, I seem to get it now. He's talking about in the ECDSA signature algorithm where a secret number k, the so-called transaction nonce (though this is the first time I heard it being called a nonce to be honest) is chosen that is used to create r and s.
Obviously that's not available in the raw transaction as he pointed out, but in another thread here I made a process for getting the nonce k from the private key, the message data - in this case the raw transaction with a few fields unfilled - , r and s:
https://bitcointalk.org/index.php?topic=5316741.msg56331333#msg56331333