His concern is: what happens if you have two transactions which have absolutely no connection but produce the same TXID? And the answer is that the second will be rejected, because of the
BIP 30 soft fork.
To be more accurate:
The second transaction will be rejected if the first one hasn't been fully spent.
If the first transaction have been fully-spent, we can have a block including a transaction with the same ID as that fully-spent transaction.