Is secureblue really more secure if browsers have to be installed in flatpaks?

Do you have the .repo files in /etc/yum.repos.d/?

/etc/yum.repos.d/mullvad.repo

/etc/yum.repos.d/librewolf.repo

Edit:

If you are a Mullvad VPN user, you could also try ujust install-vpn and select mullvad, which installs Mullvad VPN and enables the repo allowing you to also install mullvad-browser.