Contradiction! I can prove the opposite.
The installation of the e.foundation applications “Apps” and “Browser” work as ‘normal’ and ‘System’ app installation - even on third party systems.
Following the discussion here, I did a quick test - on three different devices [amr + amr64] and two different operating systems: LineageOS Fork DivestOS Mobile - A privacy focused Android distribution - and StockRom Android 7.0 .
The installation of the e.foundation apps & browsers was installed subsequently, i.e. into working systems, without having rooted the devices:once as a system app via adb commands, the other two times as ‘normal’ user app via adb install. Both applications work perfectly on all systems.
Just try it and you will see how easy it is!