It used to be incredibly common for production APKs to contain Java debug info (line numbers and variable names). IIRC, Android Studio now sets up the Release builds to strip this out and do basic ProGuard optimizations, but if WhatsApp was migrated from an old build system or something, it could easily be missing this step.
I was under the impression that it's now no longer possible to upload an APK that has been built in Debug Mode to Google Play. I don't know if other app stores (i.e. Amazon App store) are enforcing this.
Debug mode and obfuscation are completely seperate concepts. You can have a debug build with obfuscation or a release build without. Google Play doesn't care if an apk is obfuscated or not.
