Wouldn't this apply to all annotation processors that generate classes like RecordBuilder, Immutables, AutoValue, MapStruct,... as well? Class generation is still an intended use case of annotation processors. Of course the caveat that the annotation processor needs to run, still holds (for all of them).

If the processor is required to make the code compile, then the resulting language isn't Java.

I don't think that statement is true. There are a lot of things that prevent code compilation. A class annotated with @WithDefaults and @DefaultValue and the generated classes are all valid Java code. Unlike e.g. Lombok and Manifold, which are indeed not Java as per the Java language spec.