I'm no expert in security, but my intuition is that the likelihood that my signed ubuntu plus gpg leak secrets is significantly lower than my who-knows-what-the-hell-its-doing android with signal, right? You gotta agree with this at least.
Snowden in particular did much dumber things than using PGP: he used Lavabit, a "secure email provider" with an architecture any expert could have told you from 10 miles away to avoid, and some amount of the Snowden/Poitras/Greenwald coordination occurred over CryptoCat, which was grievously broken in several different ways --- it is almost certain that every message exchanged between those parties on CryptoCat has been recovered by SIGINT agencies.
People wonder why crypto engineers on message boards are so shrill about this stuff. This is why.
If anyone wants to tear out excess hair, watch Citizenfour's scenes with Snowden and the rest on their computers. The main argument against the omniscience of the intelligence agencies is that they didn't walk into that hotel room on the first day.
Besides technical issues, watching Snowden try to get Poitras and especially Greenwald to adopt security based on operational discipline and CLI *nix tools is both hair-raising (easier to tear out that way) and laughable. Clearly he never had to work with end users. IIRC, in his initial instructions to Poitras he tells her 'if you have trouble setting up that encrypted connection, just ssh to the ip address and use the tool of your choice.' (If you don't understand why that is laughable, you haven't worked with end-users either.) Greenwald seems completely at a loss - 'I made my password (to the data on which Snowden's life and the whole news story depends) simple because I have to use it all the time - is that ok?' Sure, why not at this point?
Experts I know recommend iOS above any other consumer platform (though that might have something to do with Apple's hardware and of course you can't use iOS without Apple hardware).
When talking with the security folks I know, this is not a rare opinion. You can't trust the source of any comms platform. So you have to reverse engineer the binaries. Once you get into that game the more important things than open source are, do you trust the team? Is the protocol open and vetted? Does it rely on 'odd' implementation details.
The idea that the nature of the software license of the OS that your comms platform is running on, is important for its security characteristics, does not seem to be something that the experts I know believe. Quite the opposite. It seems to be a signal of people who don't do this sort of research for a living.