Why do swords play just as much an equal role defensively as shields when using both?

[Nathan Ross]

whether the ways of fighting that we can learn from later European traditions are ways of fighting which people in the ancient world would have recognized.

Quite so. The late medieval longsword or sword and buckler duelling practised by modern HEMA groups is quite different to the traditional Roman fighting style, with large scutum and short gladius. There's over one thousand years of arms development between them.

Ancient battles were mainly fought using spears. The scutum was an offensive weapon, but the gladius would have been a very poor tool for parrying or fencing.