I also asked her, and this is what she replied:"Apologies if this has caused confusion. You can submit .playground files that work with Xcode 11.4.1, or .playgroundbook files that work with Swift Playgrounds app v3.3 on Mac or iPad."You can find the Tweet on her profile!So this means we can for sure use playground books!

I totally agree with you, and I have been thinking the same thing! I feel like the Apple employee who has commented on these issues before is implying that the Author Template should not be used to create a Playground Book, but others are okay.I totally agree that if we are explicitly permitted to use Starting Points, that includes Starting Points which are Playground Books!