That targets only Android 7 and above (and even then a subset of the features). If I understand correctly, this actually allows you to target any Android version at or above SDK 15 (Android 4.0.3), because it monkey patches your code to replace part of the standard library with external libraries. Then, when you decide to deprecate Android 6 support one day, you should be able to smoothly transition to fully native Java 8.

Kind of.

They are supporting Java 8 language features depending on the Android platform version.

Some Java 8 library APIs have been adopted, but require the latest Android versions.

Finally, the JVM bytecodes like invoke dynamic and method handles aren't supported thus making libraries that use them, unusable on Android devices.