As person who does not use OBS.
"If OBS isn't good enough for Fedora" - fanboyism is never good. If OBS has issues in development then what? What would you do if it stops updating Qt permanently? Think not let emotions act.
"works great" - doesn't mean it is secure.
You can write application that works great and is swiss cheese from security standpoint.
You can write secure application that works like nightmare.
"inappropriate" - why?
If it is statement of fact then it can not be inappropriate.
Also mind you OBS blocked the issue about fact that they use EOL qt on github - this does not look to me as good project.
"the Fedora-packaged-flatpak breaks" - is it broken? Because no one even speaks about real state of package! Or by "broken" you mean - does not have functionality I want! Or it uses Qt version which breaks the application!
Because In first case that not breakage - that's loss of functionality and if motivated by legal reasons - I can understand (not approve since US software patents are from my perspective idiocy), if motivated by security I wholeheartedly approve - because you are shooting messenger(fedora) of bad news(OBS bad practices) here.
In second - Qt is broken so send regards to them and their policy:
Update it so often to make GPL/LGPL version as miserable as possible.
Which they then use to sell companies the LTS versions under proprietary license.
I agree with breaking (it is good feedback about software state) to modernize dependencies - but then again I'm using Arch so…