I suspect Five Eyes already has backdoors into Apple and Google and can remotely push compromised updates to specific targeted devices. How do you know the version of Signal you get from the App Store is the same as everyone else? Australia can compel engineers to implement backdoors, they can't refuse or even talk about it due to gag orders. They can then share that info with Five Eyes to bypass their own laws like they did with ANOM. Australia ran the operation specifically to bypass stronger privacy laws in the other countries. The Snowden leaks with PRISM, showing governments getting data straight from Apple and Google servers. Australia's latest encryption bill lets them force companies to give access to encrypted comms. Seems pretty plausible to me.