Yes, there's a legitimate risk (and accompanying threat model) when trusting package repositories. But I don't understand the specific threat model that involves not trusting Signal's package repository while (1) trusting a random third-party package that (2) just redistributes (in the best case) the official binary.
I trust Signal, the company, as an author of specific type of communications software. I hesitate to trust them with root on my systems. The company and their intentions are, for the best of my knowledge, benign - but I have seen far too many well-meaning packaging snafus over the past 25 years to add even them to my sources.list.d; and in fact, I believe that with the actions of the company over the past ~three years they have squandered lot of the goodwill they had built up. I'm sorry to say, but the theme to me has felt like one of miscommunication combined with a lack of foresight.
I do trust they have had good reasons for everything. But optics are important, and for stewards of such a critical piece of software Signal have come up with questionably announced surprises. In a domain where boring is the characteristic everyone looks for.[ß]
A bit more context. As of now, there are only two third-party APT repositories that I can stomach. The official Postgres repo, and the Deadsnakes PPA. Both are maintained by the actual package maintainers, so they benefit from the assumed baseline and robustness.
ß: btw, I understand the SMS stuff. From an engineering effort perspective it makes sense, given what shitshow the SMS/MMS protocol stacks are. And with RCS, future integration would not be guaranteed at all. But it still came as a surprise.
Doesn't matter who provides the repository, because ownership can change. Even if you trust the current owner to be diligent and fair you have no control over what a future owner does. (Incidentally, buying widely used but badly maintained browser extensions and using them to distribute malware is a real thing.)
For example: the software everyone cares about is in a repo. The company uploads a version of a core library, such as glibc or libstdc++ in their repo and set it up with a very specific version number. Then they wait a bit, and upload a version of the much-cared software that declares a dependency against the specific library version. When the end users upgrade their packages, they also get a compromised core system library and all of a sudden ALL the software on their system is affected. The software you added the repo for has not been tampered with. But a backdoor from the core library is now present everywhere.
I worked with Nokia and Intel before and during the Maemo/Meego fusion. Back in 2010/2011 I had an early draft patch against libzypper that would have allowed to set certain package priority levels behind a fixed signing key, and made the above attack impossible. (Others were looking at how to confine the install scripts.) I went to a holiday in January 2011 and during that time, Elopcalypse happened. When I came back, the project no longer existed and my RFC patch never saw the light of the day.