That would imply an unresponsive flow (or a large aggregation of flows in one queue, which would require a high drop rate to respond).

Indeed, the Codel algorithm in itself doesn't guarantee any latency bound.  Only a hard limit on queue length can do that (or forcibly dropping packets with sojourn times above the limit).  Practical qdisc implementations do have such mechanisms.

Flow isolation helps in such circumstances, especially if the qdisc drops from the longest queue on enqueue overflow.

- Jonathan Morton