Playing music doesn’t require unlocking though, at least not from the Music app. If YouTube requires an unlock that’s actually a setting YouTube sets in their SiriKit configuration.

For reading messages, IIRC it depends on whether you have text notification previews enabled on the lock screen (they don’t document this anywhere that I can see.) The logic is that if you block people from seeing your texts from the lock screen without unlocking your device, Siri should be blocked from reading them too.

Edit: Nope, you’re right. I just enabled notification previews for Messages on the lock screen and Siri still requires an unlock. That’s a bug. One of many, many, many Siri bugs that just sort of pile up over time.