It did bounce, it bounced the Tolarian West that he used to tutor for another bounce land. Yes he could have just bounced the growth chamber, but initially he was going to tutor for a pact
You played so wrong that it made me sick, If you had made a pact and gone to get Azusa you would have enough mana to keep Colossus alive and the ability to put mana and draw cards puts Valakut's trigger ability to be resolved soon after the last land you deal 3 damage for each land that is placed on the battlefield, not just for the 6 land.
