What is not a myth, though, is that if you think you have patched some bug or hole in a system library, think again, because flatpaks are a distro in your distro (old yo dawg meme slide follows), sometimes a few even. And their update cadence is not the same.
Yes, it means that sometimes they will be ahead on patches, why not.
So at the very least, two systems on your system. Given that there are hacks to share some of your fd.o stuff, let’s say one and a half.
If you test it with VMs you can check it out for yourself: https://www.ypsidanger.com/wasting-disk-space/