Read their comment and I'm left scratching my head. Their role in security with the straight android phone (not the /e/OS version) is simply pushing security patches as/when they get them from the Android team, as they're using straight Android. Security is handled by Google for Android, not them. When it comes to /e/OS, no idea how good/bad it is, but apparently Graphene has some beef with Murena (the people who make it), at least according to their comment.
Not at all knowledgable about mobile kernels and drivers to comment on the rest of it. I do know Fairphone 5 uses an unusual CPU normally used for SoC as that was the only CPU that was both good enough to run Android reasonably while simultaneously providing very long-term driver updates (they're aiming for a minimum of 8 years of updates).