Utterly absurd. I foresee horrible situations where couples have to forgo having children for fear of not making the maintentance requirements.
Why would they have to forego having children?
It's saying that if the child has British citizenship (which they normally will have if one parent is British), then they won't have to show any extra maintenance funds for them - only the minimum amount of £18,600 for the couple.
However, they will need to show the extra maintenance if the applicant has a non-UK citizen child from a previous relationship.
Under the current rules, you have to show maintenance for all dependent children, whether they are UK citizens or not, so this rule actually seems to be an improvement.