Second-Social gathering Transaction Malleability in guide “mastering bitcoin” third version

In guide “mastering bitcoin”, third version, web page 136, there’s a assertion as under.

“Tx1 spends the second output of Tx0 to 2 new outputs, one to Alice for her share of the joint funds, and one to Bob for his share. Alice and Bob each signal this transaction earlier than they signal Tx0.”
My query is why Alice and Bob can each signal transaction tx1 earlier than they signal Tx0? as a result of tx0 id(txid 0) is the a part of knowledge to producing tx1 id.

i do know 2nd malleability is about random included by ECDSA signature, however i simply wish to know if the assertion above is correct?

reference hyperlink:
https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch06_transactions.adoc

Thanks!