More info further down the article: > ... adding a small amount of security metadata on top of each APK to verify that it was officially distributed by Google

More info further down the article: > ... adding a small amount of security metadata on top of each APK to verify that it was officially distributed by Google Play. ... The metadata we're adding to APKs is like a Play badge of authenticity for your Android app. > > ... the small metadata addition, which is inserted into the APK Signing Block ... Yes it looks like an additional signature. > ... this metadata will enable new distribution opportunities for developers in the future and help more people keep their apps up to date. This suggests that you may be able to distribute signed APKs outside the Play store (e.g. host the APK yourself and email a link to beta testers?).