It's the other way around tho. Gunboat diplomacy for latam and the Marshall plan for Europe established the material groundwork for American culture to grow in.

Exactly. I think it's also worth noting that it's not as if soft power isn't material. Soft power is not idealism it's just not as straightforward as direct military might. What Alaskaball said is true of course, but those things also feed back into soft power which in turn makes US hegemony, military dominance, and imperialism (like gunboat diplomacy, Marshall plan, etc.) more accepted and even supported by the vassals and the imperialized populations. Not unlike how base and superstructure both form a reinforcing and reproducing feedback loop with each other.