Security is a policy-driven spectrum of considerations and solutions. GrapheneOS targets very specific threat models, which is not possible with Fairphone hardware/BSP. Whether it makes it not secure for your own use cases, it's up to you to decide.

Case in point: re-lockable bootloader requirement. Not everyone is a target for an evil maid types of physical attacks or possible state actor pressure. But when you actually need it, it's not negotiable.