More than anything, it’s a vocabulary to talk about the thing you’re doing with others.
I doubt there is any composer that actually is relying on music theory for their compositions. They may occasionally have something finished, but 'it doesn't sound right'. Then go back to music theory and say, oh, this chord is wrong. I can fix that chord because I know music theory and come up with something that works. But they're not doing that at first. They're composing from their unconscious mind.
Would you expect an author to disregard all of the conventions we've discovered and codified through specified language because it's just "more natural" if they let it flow out of them? No, I don't think you would. You'd find it difficult to read and interact with, both as a reader and a fellow writer. Music is the same.
Music theory is just the natural language definitions of the sounds we hear. A composer is absolutely thinking in those terms and using them to construct the piece they hear in their head. It gives you a way to communicate, with words, the sounds you are hearing in your own head. It does not constrain music in any way.
No composer I know thinks in the terms of music theory. They often use a chord progression that music theory describes why it works, but they're not thinking of it in terms of music theory. They're thinking in terms of chord progression, just like the composers of thousands years ago were thinking of their chord progressions the exact style changes and so a thousand years ago they went to thought in terms of modern chord progressions but they were still thinking in similar terms.
Broadly completely untrue.
If you're trying to say that humans were making music before we had a theory for it - absolutely. But every advanced musical tradition on the planet (including Western, but also Indian, Chinese, etc.) has developed alongside a system of analyzing that music, and for very good reason: It's a lot easier to teach music when you can actually talk about it.
I promise you, every single competent composer and musician on the planet - from every culture - has a fairly firm grasp on music theory, in whatever flavor is relevant to the form of music they practice. Every Indian musician who plays classical Indian music knows their ragas. Every Western musician can navigate the diatonic scale.
Sure, at some level of expertise you’re not sitting there saying “oh, I should name variables sensibly and encapsulate functionality because computer science says to”; they are doing those things because with training and experience that becomes innate and accessible in flow state.
But the fact that theory can become ingrained does not mean one isn’t using theory.