I disagree with that. IMHO the best possible people to create a package for an application are the original developers of that software. If that software is proprietary, that also happens to be the only party that can legally do that anyway. Because it typically requires access to the source code and software redistribution requires permission.
So, the model you mention only works for open source packages. And I would argue that even in the case an app is 100% open source it's a bad idea for somebody not affiliated with the core development team to be second guessing a lot of things about that application.
It results in a lot of issues that aren't necessary. Like needless lag between developers releasing new software and some third party doing whatever uninvited tweaks they think are necessary, or adding their own bugs and new issues.
It's why I always install Firefox in tar ball form straight from Mozilla for example. It updates itself as soon as developers OK some patch. This happens a lot and mostly for security and stability reasons. I want those patches when they release them. The things external distribution maintainers do are redundant. I trust Mozilla to do the right thing and be the most clued in about any issues regarding their own software. With proprietary stuff, I just want stuff to run with a minimum of hassle.
Flatpak is trying to do too many things. It's trying to emulate an appstore. I don't necessarily like app stores. They are gate keepers. What do developers on Windows and Apple do? They put binaries on their own website. You download them. You install them. And then they run. Downloaded apps have the same rights as apps provided via app stores. The app stores don't repackage the app, they merely distribute them. It's an add on service. An optional extra. All the essentials that provide security are baked into the OS and the application package. There are a few mechanisms that windows and mac provide to make things secure. Binaries are signed, the OS has a permission model for things that need that (screen sharing, directory access to certain things, using the webcam, etc). That's the right model. That could work for Linux as well. It shouldn't require taking control of distribution or packaging by some third party.