Questions about secondary transaction malleability explained in the book “Mastering Bitcoin”

On page 136 of the 3rd edition of the book ‘Mastering Bitcoin’, there is the following phrase. “Tx1 uses the second output of Tx0 for two new outputs: one to Alice for her share of the common fund and one for Bob’s share. Alice and Bob both use this before signing Tx0. “I sign the transaction.” My question is why can Alice and Bob sign this transaction before signing Tx0? This is because tx0 id (txid 0) is part of the data that creates tx1 id.

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