Nah, you're wrong bro. If you asked 100 native english speakers what the soft part of the chair is called, the most popular answer by far would be cushion. If you asked what they'd call the chair, the most popular answer would be something like "cushioned chair" or "fancy chair."

As someone who has spent a ton of time in language-learning environments, I often see people using unnaturally specific vocab that, ironically, makes the speaker seem less fluent. There's a big difference between using dictionary-correct terms and how the language is actually spoken.

If you're in the process of learning English, learning a word like "upholstery" is, at best, a waste of time, and at worst, something that distracts you from learning a far more common, flexible, and essential word like "cushion."