This is part of the (spiritual) lineage of Meego/Maemo, it's much older than GrapheneOS and the latter is older than Android itself

Anyway, it's as secure as any Linux distro as it uses the same standard stack as servers and desktops and does sandboxing[1], which is also really nice from a development perspective. You can harden it like you would a Linux box using standard Linux tools + kernel features.

[1] https://en.wikipedia.org/wiki/Sailfish_OS#Software_architect...