I have a theory. Denis is immune to makima's control, or more specifically, have an ability similar to Makima's control.

To begin with, it is worth explaining where he has such a feature in general. Let's remember the Doll devil who devoured a piece of flesh from the Darkness devil.

She became stronger and smarter as well. She also acquired the traits of a Darkness devil and gained the ability to regenerate in the dark.

All this she acquired by devouring the flesh of a stronger devil.

later in the story, the Pochita tells that devils that eat the flesh of other stronger devils become stronger, and from the experience of the Doll devil, they also acquire some features from those they ate.

But here there is a small nuance. The Doll Devil itself has no main body. The Doll devil is rather a kind of mind controlling a HUMAN.

And as we can see, the person who ate the flesh of the devil became stronger.

But there are nuances here.

The person must be under the influence of a Devil. His body must be different.
Flesh must be consumed while the Devil is still alive. Otherwise, every devil would want to eat others to become even stronger. And the devils aren't even interested in that right now.
All these facts lead us to the next.

Denji may have gotten some powers from Makima when he ate her.

As evidence that Dennis can do this is the scene with Power. There he absorbs her blood and then creates a chainsaw from it and even uses it as an artificial heart.

Perhaps his strength has something to do with his charisma or the fact that even though people are no longer afraid of Chainsaw Man, he is still strong.