Earlier quoted context omitted.
He never said it was. Google services aren't isolated like normal apps either, as far as I know they can access other app's data (when installing cyanogenmod, I had to install google apps in some weird way from the bootloader because it has to change protected things on the phone). His point of requiring google's stuff to be installed is valid, even if he wrote it thinking payload gets sent over it.
Even if you don't install Gapps, large parts of Android/CyanogenMod are from Google as well. How does installing Gapps make the security worse?
Stock Android does not, by inspecting network traffic, contact Google servers.
Google play services and other GApps, do, and they can be exploited in this traffic, or told by Google to activate other backdoors.
Signal with GApps, Google can know which phones, and which users, are using Signal, thats a security vulnerability. Google can infer from their Google-messaging thing, that notifications are sent, and have a high probability of knowing if it is to Signal. Who talks when is leaked to Google.