He puts it a bit too general, but against full plate armour, swords are nearly useless. In the late Middle Ages, when plate armour became increasingly common, people started using weapons with better armour-piercing capabilities, like maces, warhammers and various polearms.
Swords are great against lightly armoured and unarmoured targets, like peasants, archers and civilians. Swords remained in use as a side arm and civilian weapon, and a popular duelling weapon, which may also be why so many medieval manuscripts are about one-on-one fights. Carrying a polearm with you everywhere you go is a lot more cumbersome than having a sword hanging on your belt.
But swords were absolutely used on the battlefield. Romans and vikings used them a lot. Knights had them of course, and even they became increasingly less useful against other knights, they often didn't really want to kill each other anyway. Pikemen generally had a short sword as backup weapon (the Katzbalger), and there's of course the massive two-handed swords used by Landsknechts to break up pike formations (supposedly; nobody knows for sure). But again knights in plate armour, you'd better get something that can knock holes in it. If you want to kill him, that it.
You might compare the sword to a pistol. You don't usually send a mass of soldiers into battle with just a pistol; you give them assault rifles (or polearms). But officers and civilians tend to carry a pistol, and maybe some soldiers have one for backup.
Swords are great against lightly armoured and unarmoured targets, like peasants, archers and civilians. Swords remained in use as a side arm and civilian weapon, and a popular duelling weapon, which may also be why so many medieval manuscripts are about one-on-one fights. Carrying a polearm with you everywhere you go is a lot more cumbersome than having a sword hanging on your belt.
But swords were absolutely used on the battlefield. Romans and vikings used them a lot. Knights had them of course, and even they became increasingly less useful against other knights, they often didn't really want to kill each other anyway. Pikemen generally had a short sword as backup weapon (the Katzbalger), and there's of course the massive two-handed swords used by Landsknechts to break up pike formations (supposedly; nobody knows for sure). But again knights in plate armour, you'd better get something that can knock holes in it. If you want to kill him, that it.
You might compare the sword to a pistol. You don't usually send a mass of soldiers into battle with just a pistol; you give them assault rifles (or polearms). But officers and civilians tend to carry a pistol, and maybe some soldiers have one for backup.