@kireato you might be right, the perspective is really bad in that one panel and you cant see any of his arms at all. I just assumed it was chomped as the mouth was open, he approached from below with left arm ahead, then just before lifting the mouth was closed (obvious as his upper body is now partially above it) which I can only imagine it did voluntarily considering his position, and the panels where he suplexes it we only ever see the right arm (and not the left one that as I mentioned was extended far ahead of him as he rushed and bent infront of him as a guard in the last panel where wyvern has open mouth).
So yes, you are likely correct now that I look closer on that really weird perspective panel, and I were probably just led astray by author slightly messing up positioning (no way he will get his chin above dragon and arms around the snout to forcefully close it when it is open wide like that. He simply lacks the arm-length).