The principles of strategy are written down here in terms of single combat, but you must think broadly so that you attain an understanding for ten-thousand-a-side battles.

It appears to me that Musashi intends his principles to be applicable to either duel or battle. There are several illustrations that imply that he is not just talking about duels:

First, in large-scale strategy, when the enemy first discharges bows and guns and then attacks, it is difficult for us to attack if we are busy loading powder into our guns or notching our arrows. The spirit is to attack quickly while the enemy is still shooting with bows or guns. The spirit is to win by "treading down" as we receive the enemy's attack.

In large-scale strategy, when you cannot see the enemy's position, indicate that you are about to attack strongly, to discover his resources.

In large-scale strategy you can frighten the enemy not by what you present to their eyes, but by shouting, making a small force seem large, or by threatening them from the flank without warning.

In large-scale strategy, it is beneficial to strike at the corners of the enemy's force, If the corners are overthrown, the spirit of the whole body will be overthrown.

I suspect that 'flanks' might be a more idiomatic translation of that last. But at any rate it is clear that Musashi does not limit his advice to individual combat.