"you can grab the apk from any number of third party download sites"

But people get it from the official app stores and for WhatsApp there's no other option (which is the main one I actually care about because that's where my contacts live).

I think some of this discussion is caused by conflation between Signal-the-tech and Signal-the-app. I could have called it the Axoltl Ratchet instead but I don't think anyone would know what I'm talking about, and anyway, I think it'd be less technically accurate as by now there's lots of stuff to do with group chats etc and I don't know what the underlying code-name is of those schemes. I normally see the whole encryption scheme just be called "Signal encryption".

If your position is "We should have infrastructure that makes it easy for third parties to audit Signal updates correspond to the source code", I absolutely agree! But we can build that with what currently exists, Signal's centralised infrastructure does nothing to prevent that.

We are in 95% agreement indeed! I proposed such an infrastructure in the article, based on threshold signatures, however, it doesn't exist today yet our industry is claiming "we use end-to-end encryption which means we can't read your messages". This claim isn't true - they can read our messages if they want to - and progress seems to have stopped after these rollouts. The claim that it's possible to have a trustless centralized infrastructure provider is a very bold one and currently it's not convincing, so to prove this is possible we need to keep things moving. There has been no attempt to make these services auditable nor even really any recognition that there needs to be. That troubles me a lot. At some point people are going to notice these claims don't add up and trust in the whole software industry may be damaged.

Edit: actually if I recall correctly Signal uses Intel SGX to make some parts of its contact list intersection system auditable. That's a great use of tech to solve some of these problems. SGX is one way to attack the centralized-but-untrusted-provider problem. I don't know if anyone is actually doing remote attestations on a regular basis though.

"Every time someone sends an encrypted message, they're relying on a huge stack of technology that's largely outside their control"

Yes, but those components come from different players, in different countries, with different governments and agendas who often do not trust each other blindly. Hence why mobile radios are now sandboxed, why keyboard apps are sandboxed, Samsung presumably watches out for Google playing naughty games with the Android source as part of their patching and distribution process, etc. Certainly carriers have a long history of treating phone makers as semi-trusted, hence the historically long approval processes.

"Signal doesn't appear to have changed the deal, so it seems like you're saying we can currently claim that it's end-to-end encrypted?"

I believe that is currently the case. I cannot prove it, nor can I prove it will remain the case tomorrow, if it is today, nor can anyone else I trust prove it. Which, logically, means it boils down to faith in Moxie and his employees, people I've never met and do not know. A blog post saying "we promise we're not logging messages" should really carry equal weight.