Why do Anchor Outputs need to enforce an nSequence of 1?
Because there must always be exactly one directly spendable output per party, in order to avoid one party to be able to &quot;exhaust&quot; the carve-out.
With two CPFP-ble outputs, one party could both create a chain of descendants transactions on the first outputs so that it meets the standard mempool limits and then trigger the carve-out mechanism using its own second output.
This way, the HTLC transactions are not included in the mempool and cannot be used to either create a long chain of descendants, or used as a carve-out transaction.
xref:

The carve-out proposal
The first mail (by Matt Corallo) expliciting the need of at most one directly spendable output for the carve-out to &quot;work&quot;.