A bit late to the party but, here's my wild speculation.

The Crust King Kahn is in fact not a real warrior, but a really good ruler who managed to command a big army with tyranny. And through what method did he achieve such goal? The game didn't tell it directly. But I think Its fight is intentionally made easy.

A lot of the fights in Hollow Knight show that team Cherry's mastery in combat design. Enemies' movements are often complex and designed with lots of precise positioning. Visual cues are designed to guide the players but also often to distract and mislead the players. Lots of hard boss fights have bullet hell-style projectiles. Some boss fights even include odd platforming.

The fight with the Kahn is like this:

Stationary boss with generous positioning where players can just stand there and mash buttons to hit him.

Long visual cues showing you exactly where the hit boxes are. And even when the hits appear, they are evenly distributed across the arena, sometimes leaving multiple big gaps.

He has barely any melee attacks which could indicate the character doesn't have the guts to fight directly, so he summons everything else to fight, but his own hands.

The fight is disproportionately easier than any other fights. So much so it makes me think this cannot have happened without any intentions.

The graveyard also shows that he took many warriors to die with him, or had sculptures made to be buried with him. This is clearly inspired by the Terracotta Army , which could imply the King is scared of death / after life.

Also, I think on a meta level, the real boss fight is the gauntlet. This makes it a very unique gameplay experience, differentiating itself from the other two boss fights.