It’s true; that’s quite possibly sufficient. But it’s not naturally true either, because fee and feerate are separate dimensions: the child could be high fee but low feerate; making it go into the same chunk as the parent could be quite expensive in that case.
If we wanted to permit this (and the “gather chunks until relay rule satisfied” approach isn’t broken in other ways), it wouldn’t be a high computational cost. The expensive part is the linearization step (10), and that doesn’t need running until a relay-acceptable collection of chunks is found