Signal is told to be a secure crypto messenger everyone can use. This means that people are installing it on a usual Android device and assume that nobody is able to read their messages without physical access.

Of course you are right, there are many black boxes in most mobile devices. But does this mean we shouldn't care about any of them? Most of these black boxes are not updated on a regular basis or the update requires user consent. This means that, if they don't allow arbitrary code execution now, it is hard to make them do evil things.

The mentioned issue with Google Maps integration (leave the GCM problems aside) however is a real possibility to silently drive targeted attacks on Signal - it's actually damn easy for Google to do this. If we'd be doing a proper analysis on all other black boxes (processor, modem, manufacturer extensions, etc), I doubt we'll find such kind of backdoors in most of them. And if I will find such a thing, be sure I'll be writing about it as well.

This backdoor is important for those that want to securely communicate and their adversary is for example the US gov. e.g. the next Snowden. Google can be forced by US agencies to use this backdoor and this renders Signal unusable for them. These people should know about the backdoor. If you know and don't care, that's your problem...