As a native U.S. speaker who doesn't think much about grammar...
...it SEEMS to me that "She already fluffed that pillow" is ALSO correct.
Question: "Has she already fluffed that pillow?"
Answer: "Yes, she's already fluffed that pillow."
Question: "Did she already fluff that pillow?"
Answer: "Yes, she already fluffed that pillow."
Question: "Do I need I to fluff that pillow?"
Answer: "No, she already fluffed it" works, and "No, she's already fluffed it" also works.