There is no exception in the App Store rules that says, "You can't do this, but if you bundle other things with it, then you can."
No. Flat out, no. This is nothing other than Apple letting another big player slide on the rules like they have done 100 other times.
The rule prohibits "monetizing built-in capabilities." I have spoken with many developers over the years, and even some folks at Apple about this from back in the days when the rule was first put into place. It's about preventing developers from trying to simply "cash in" by unlocking what Apple has already provided.
YouTube Premium isn't an "exception" to the rule. The rule doesn't apply in this case, because Google is not "monetizing" picture-in-picture. That is to say, they are not making money from that feature. That's what the rule is about — Apple doesn't want developers profiting from features that are supposed to be available as part of the iPhone or iOS. However, as others in this conversation have said, nobody is paying for YouTube Premium just to get PiP.
Similarly, there are many apps that only allow access to Push Notifications to those who subscribe to a paid tier. Push Notifications are specifically mentioned in the rule as something you can't charge for, and people complained about this over a decade ago when Apple introduced Push Notifications in "iOS" 3.0 (or "iPhone Software Update" as it was called back then), and apps began unlocking Push Notifications with in-app purchases. However, as long as the developers weren't charging only for Push Notifications, they were fine. In most cases, they were charging for the information behind the push notifications — things like "breaking news alerts," or "instant weather alerts."
Popular apps like Carrot Weather, Todoist, and Apollo all come to mind off the top of my head as apps that only provide push notifications to paying subscribers. In fact, Apollo is a perfect example of how Apple's rule works — it was originally rejected by the App Store Review team until the developer added themes and icons to the subscription to make it about more than just the notifications.
Again, I'm not trying to give Google a pass here, and I'll be the first to call Apple out for those places where it has let the big players slide by, or created special side deals. It's just that this isn't one of them; YouTube is a visible app that's doing this, so it gets a lot more attention (and well-deserved frustration for how stupid the whole situation is), but it's not hard to find hundreds of apps that only make "built-in capabilities" available to paying subscribers — it's just that those capabilities are inherently tied to other things that come with that paid tier.