It's not about "killing machines", but excellent application of Infantry tactics that defeated a numerically superior foe.
Focusing on just one moment of the battle to prove your point is misleading.
If that's your justification for not giving ICA to Portugal then no country should have Military bonus on traditions.
Or were English soldiers killing machines to justify having 10% ICA?
North Africa has Morocco, that has the same tech level, not to mention that the states from east africa and asia were not technological backward nations.
In game terms, without babysitting Castille, Portugal does squat in North Africa, if allied with England, France just farts in their general direction and they are dead, Kilwa and their 2 allies in the region just wreck Portugal navy, and not gonna even mention India where the local powers can just easily roll over Portugal.
That's why I argue that Fort defense should come as second idea, and Legacy of the Navigator should buff the Navy durability, and third idea should buff the Navy a tad more, while having ICA in traditions.
Valverde:
Nuno Álvares Pereira then ordered his army to form a square with the baggage on the center, and impetuously crossed the Castilian forces, which attempted to oppose him. Upon reaching the river shore, Nuno Álvares Pereira ordered his rearguard to protect the baggage and fight the enemy, while with his vanguard he crossed over. The Castilian forces on the opposite shore, numbering about 10,000 men, unsuccessfully oppose their landing.After placing the Portuguese vanguard in position, defending the shore from the Castilians, Nuno Álvares Pereira again crossed the river to reach his rearguard, which was under a rain of arrows launched from the Castilian side. The Constable of Portugal noticing that the Castilians had used all of their projectiles, ordered an attack. Nuno Álvares Pereira himself, seeing the banner of the Grand Master of Santiago, fought his way through the Castilian army until encountering him, and after a brief duel, the Grand Master fell mortally wounded. With his fall and the overthrow of his standard, the Castilian army demoralized and was soon broken and in disorder, and could not stop the Portuguese attack, being quickly and totally defeated.
Aljubarrota:
The initiative of starting the battle was with the Castilian side. The French allied heavy cavalry charged in full strength, in order to disrupt order in the enemy lines. Even before they could get into contact with the Portuguese infantry, however, they were already disorganized. Just as at Crécy, the defending archers and crossbowmen, along with the ditches and pits, did most of the work. The losses of the cavalry were heavy and the effect of its attack completely null. Support from the Castilian rear was late to come and the knights that did not perish in the combat were made prisoners and sent to the Portuguese rear.
At this point the main Castilian force entered the battle. Their line was enormous, due to the great number of soldiers. In order to get to the Portuguese line, the Castilians became disorganized, squeezing into the space between the two creeks that protected the flanks. At this time, the Portuguese reorganized. The vanguard of Nuno Álvares Pereira divided into two sectors. John of Portugal ordered the archers and crossbowmen to retire, while his rear troops advanced through the space opened between the vanguards. With all his troops needed at the front, there were no men available to guard the knight prisoners; John of Portugal ordered them to be killed on the spot and proceeded to deal with the approaching Castilians.
Advancing uphill with the sun on their backs, squashed between the funnelling Portuguese defensive works and their own advancing rear, and under a heavy rain of English longbowmen's arrows shot from behind the Portuguese line and crossbow quarrels from behind both the Sweethearts' and the Honeysuckle wings on their flanks, the Castilians fought to win the day. The Castilian knights in the main body were forced to dismount and break in half their four-metre-long lances in order to join the constricted melèe alongside their infantry.
At this stage of the battle, both sides sustained heavy losses, especially on the "Ala dos Namorados" where the Portuguese students became renowned for holding off the heavily armoured knights of the Castilian wings who, still on horseback, attempted to flank the Portuguese lines. A similar attack was more successful on the right "Honeysuckle" flank, though only briefly and late in the fight.
By sunset, only one hour after the battle began, the Castilian position was indefensible. When the Castilian royal standard-bearer fell, the already demoralized troops in the rear thought their King was dead and started to flee in panic; in a matter of moments this became a general rout where Juan of Castile had to run at full speed to save his life, leaving behind not only common soldiers but also many still dismounted noblemen.
The Portuguese pursued them down the hill and, with the battle won, killed many more while there was still light enough to see the enemy.
My personal favourite from one of Portugal less known Commanders:
en.m.wikipedia.org