But while he was looking for his weed, he figured it all out before he asked Saul. THAT'S the stretch.
It's not that huge a stretch considering that Jesse immediately suspected Huel lifted the cigarette at Walt's behest to poison Brock and put a gun to his head over it. Walt was able to talk him out of that and pin the blame on Gus, but Jesse's been doubting Walt all along, because ricin wasn't used to poison Brock, Walt conveniently found the missing cig in the Roomba, when Jesse knew that could not have been where it ended up per the reason given in my prior message, and because Walt's protestations that "he'd never hurt a child, but Gus would" fell flat with what happened in the desert and Walt "whistling while he works" afterwards. So now Jesse finds his pocket picked again after tussling with Huel, with Walt again being the one to benefit from it. It's not a stretch at all that he finally knew
; just a couple hours earlier, he had melted down again and begged Walt to stop lying and admit his real motivations, and Walt responded by hugging him, so the distrust was all very much fresh in his mind as he was waiting to disappear to Alaska so Heisenberg wouldn't have to kill him.