Reciting thana in place of dua-e-qunoot

On pg 217 beheshti zewar, masla 32. Mention is made of a person who instead of reciting dua e qunoot in witr, read thanaa, then later read the qunoot, no sajdah sahw is required. If he doesn't read the qunoot later, ie just reads thanaa or something else in the dua e qunoot, will sajdah sahw be wajib? The kitaab mentions dua qunoot is wajib. Like surah Fatiha. If one reads something else instead of Surah Fatiha sajdah sahw is wajib. Please can mufti sb explain why for the qunoot it is not the same?