Considering that Portugal is in a latitude that will be habitable, like we can see by the states in Italy and the Balkans, the nearly 900000 survivors (around 20% of the pre Fall population) will be in a reasonable condition to survive (even considering they will experience some bad times), plus they can later rebuild things, and since they can move to the lower more fertile grounds after a while (and most urban centers are gone), and the percentage of farmers in the population is larger, they will be back (I just can't tell whether as reunited with Brazil or independent (probably with German kings), for more details, please see these posts).So what your saying is that most of population is dead, most of bureaucracy and government has been decapitated, most of the merchant marine is destroyed, and its principal ports are devastated. Given this the highland minority will be able to restore order in time, to reconstruct their ports and merchant marine, in order to evacuate their population to a hostile territory in Africa where they had only a loose control of the coast.
I'm not buying it.
Keep also in mind that the Portuguese colonies can also send ships to collect people, and the map refers to a period long after the fall.