I use flatpak on my system for the fact that packages can be had there I can't get through my package manager easily and officially. That's the only selling point!
Yes, that's kind of true. You can get newer versions of things slightly easier. It can be major point if your distro is getting quite old (like RHEL or even xenial), but if you keep up with a 2 year cycle of distro updates like ubuntu LTS, you rarely hit that scenario.
That's definitely true! It's also good for smaller distros with smaller package managers. It's easier for a dev team to support just Flatpak than to bundle/petition for pacman, apt, yum, eopkg, and so on.
That's a pretty huge selling point - I think it was the main reason for creating them.
The flaw this post seems to highlight is that they shouldn't claim that they are sandboxed when they aren't. But even without sandboxing they're still a good thing.