What you actually described, however, is websites and apps reporting information about their content to the OS. That would indeed give more power to parental controls. But what's being legislated is reporting age range to platforms.
I also don't see how it takes anything away, you could still set stricter policies with those tools, or more mild ones if you set the age to 18.
Maybe I actually like the defaults for some age range blocking and want to make an exception. So, what, parental controls that would like to support this now must implement lying to each app or website individually?
We have age rating for movies and games and the labels are very easy for parents to discern what to buy for their kids. It would be easier to set preferences on an accounts like steam to filter out games with nudity and brutality, than to let steam know that the user is a 14 year old child.