For info on B757/767/787/777/747 you still have full CAT3B autoland capability with total FMC failure !
Not quite...
In the 744 there will be no autothrottle, so no Cat 3 capability for most operators. In any case, it is not "full" Cat 3 capability.
As for DAL167, while it is POSSIBLE to continue from SEA-NRT with a dual FMS falure, I would not consider it smart. Pilot workload would be increased significantly, including a HUGE workload increase in the terminal area at the end of a long flight. With so little tolerance for deviation from the norm these days, returning to SEA was a good move.
Also, while it might not be a true "emergency" situation that had high risk to life & limb, declaring an emergency is often the only way to get priority handling so fuel dump and return can be accomplished.