While I agree, that paranoia is good when using technology, flatpak is quite nice thing in my opinion. Yes it is technically Red Hat thingy, but not only and many things we use are from Red Hat (or they people are involved in upstream), but I personally think it is good. Many projects would be not as advanced, nor stable without them.
Of course, usually their primary goals are to fulfill needs of their distros or customers, but still, they are writing open source code, which we can read, analyze and use. It is not evil company in my opinion, I think quite opposite. They are giving back to communities much more, then other companies. I says that despite that I personally was never big fan of they distros.
Flatpak is just kind of desktop oriented containers, but I personally think, it adds a lot of value, allows nice integration (exported mime, icons, granular rights control …).
And I think, that it is great solution for muslc distros. It allows us easily use proprietary stuff, or app, which are complicated to compile with muslc.
Another great thing is, that usually upstream is managing their apps in flatpak, so they are actual and usually well behaving.
And yes, there are alternatives, but nothing is so easy for end users as flatpak.
Just my 2 cents.