Fairphone 3 has an end-of-life Linux 4.9 kernel which hasn't been receiving updates for multiple years. They weren't shipping kernel updates even while they were available. Fairphone 4 and Fairphone 5...
As you already said, using an alternative OS won't significantly improve security because it won't provide missing the many missing firmware, kernel, driver and HAL patches in practice. Using LineageOS would mostly mean continuing to get AOSP userspace patches after end-of-life. The firmware, kernel, driver and HAL patches are very important and receiving only the AOSP userspace patches doesn't really achieve all that much when many major Linux kernel holes aren't patched.