Hoeg Law weighs in on Legal Eagle's analysis here. The crux of it is that YouTube could have offered a mixed-audience identifier if it did require an age-gate (just one sign on) to view YouTube videos, which could be done through a normal Google account. Content that does not target kids but that might inadvertently get directed to kids ("family friendly") arguably needs this kind of verification before data is collected from viewers. YouTube doesn't want friction for visitors just wanting to browse (neither do informational websites). It could otherwise assume that all visitors who didn't sign in are under 13.
Here is YouTube's official comment to the FTC today.