As a user and someone who knows nothing about flatpak other than "it can install apps" what you said is true. I use flatpak and appimage because they allow me to use software without cluttering my /bin and other root subdirectories. And also because I can install with them software that my distro does not include in its repositories. All resides in my home and it's easy to remove without leaving any traces. I didn't know flatpak apps were supposed to be sandboxed until I read this article when it was posted a few years ago. Whenever I want to sandbox something I use firejail.