Does the resting sit (Jalsat al-Istiraha) substitute for the sitting for separation if one mistakenly believes they performed two prostrations?

Primary text

If one believes they have performed two prostrations and sits for the resting sit, this does not suffice for the sitting required for separation because the resting sit is a Sunnah act, and a Sunnah cannot substitute for an obligatory act (Wajib), similar to omitting a prostration in a Rak'ah and then performing the prostration of recitation (Sujud at-Tilawah) instead.