The fact that the website hasn't been updated since 2020 and still has an open CVE shines the light on Flatpak's attention to security.
Regarding point 3, if that's true, then why are all of the most-dowloaded packages on Flathub mislabelled as 'sandboxed' when they have full write access to a user's home directory? That isn't a sandbox.
Flatpak currently has a 7.2 vulnerability that has gone unaddressed since 2017. The maximum vulnerability rating is a 9, so this is quite major.