Your earlier comment complains about pulling images securely, presumably meaning signature verification, which I believe Flatpak does.
The report you linked is about tying downloaded sources to their author using public key infrastructure, which is a different issue. APT and dpkg don't do that, either. (I know this because I build and publish with those, too.)
Can you name a packaging system that does? I can't. I would like to see it (along with reproducible builds) integrated into the software ecosystem, and I think we're moving in that direction, but it will take time to become common.
I have my own criticisms of Flatpak, mostly regarding the backwards permissions model (packages grant themselves permissions by default) and sloppy sandboxing policies on Flathub, so I caution against blindly assuming it's safe. But claiming that it doesn’t support signing of releases is just plain false.