How does him taking that ricin make a definitive connection it was Walt when it wasn't the ricin that almost killed Brock?
Because Jesse had a gun pointed at Walt's head and all he needed to do was plant a seed of doubt. Jesse went to Walt's house and had it figured out- that Walt had Saul lift the ricin- but Walt convinced him otherwise. Then the ricin is magically found when Walt goes to talk to him.
Jesse connected the dots when Huell lifted the weed, which is exactly how the ricin was lifted. Saul confirmed that he took the ricin on Walt's order as well. Remember, Jesse always had the suspicion that Walt did it, he just finally got the proof.