I doubt seriously that any trapeze mechanism would attempt to also project the missile forward out of the side bay, I believe stanisslas picture does illustrate a pylon launch? since we've never seen any detail of the side bay mechanism on the Su-57? (odd isn't it) I tend to lean toward our friends observation?Is there even a hard point near the side bay area? It looks very much like the missile was launched from the side bay from Deino's screen cap.
However, I am confused by how far the missile sticks out of the front of the bay. Is there a trapeze structure that juts it forward when deploying the missile?
Does someone have an official source leading you to believe it was a side bay launch...