Everything in the article still applies to Chromium. Independent parties packaging it insecurely doesn’t affect the security of it upstream.
Looking into Flatpak more, it appears they disable Chromium’s Layer-1 sandbox and replace it with the much weaker Flatpak sandbox for seemingly no reason: https://github.com/flathub/org.chromium.Chromium/blob/master/patches/0005-flatpak-Add-initial-sandbox-support.patch
I’ve also heavily updated my Firefox article recently and I’m going to push another update soon.