Is what happened to the worshiper, namely, Satan's whisperings to him that he prostrated to a pillow in front of him, considered disbelief (kufr), even though his intention was to prostrate to Allah alone, and this fear and whispering continued throughout the prayer?

Rest assured, you are upon Islam, and no act of disbelief has emanated from you. What you are experiencing is a whisper (waswasah) from Satan. Do not pay attention to these whispers and strive against them, for they will not harm you as long as you dislike them and seek to rid yourself of them. You are on good, and you will be rewarded for your struggle against them.