Vistaus Flatpaks are the future of software distribution on Linux, especially for companies looking to port their applications and do not wish to be locked down to a distros repository, or managing a bunch of different packages.
Flatpak ensures that an app will have what it needs to run on any distribution that supports Flatpak and can be easily updated, whereas AppImages require hacky solutions or 3rd party tools to manage. Flatpak is also far more popular and better supported than AppImage. Some AppImages even require different versions of things like FUSE, (ever had those libfuse.so.2 errors?) and then you have to through the process of installing things needed for the AppImage to open, then you have to Chmod it in most cases. With flatpak, it's just "flatpak install *.flatpak" and you're done. It also handles .desktop files where for AppImage you again need 3rd party tools, or do it your self by hand.
Flatpak also offers sandboxing and security isolation, which is pretty nice for a browsers.
Managing a Flatpak is much easier than handling AppImages.
I think making it a flatpak is the perfect choice.. at least IMO.
They could do an AppImage and roll that up into a Flatpak pretty easily, then provide both if they choose with minimum work. I know Duckstation use to do that in its scripts.
Hope that didn't sound too aggressive, lol I don't really have a super strong opinion on how they package it as long as it isn't snap. 😛