The title of the section is using Java 7 source compatibility, so I don't think it is about compiling to be compatible with 1.6, it's compiling to be compatible with 7. Realistically, Android code doesn't get run on a JVM anyway, so the whole thing is kind of silly. If their translation to DEX byte code supports only 1.6 class files, then it supports only 1.6, but clearly it supports 7 now.
Try-with-resources requires library compatibility. All other Java 7 features can be expressed using Java 6 bytecode structures, so it doesn't surprise me that they're allowing them in pre-KitKat projects.
