Yes, even Apple with its practically infinite resources took 14 years from when it acquired Infineon's mobile chipset unit to launching its C1 modem. So much of the telcos' allegedly open protocols are actually implementation-dependent that it takes a lot of testing on actual mobile networks to validate interop.
Nobody, including Graphene, is getting away with building their own modem firmware. The reduced blobs are on userspace and some HAL components.