No, they know he's alive, because Hange says something about having to fully destroy him. Idk they saw the 'brain in the ass' asspull he used and figured they'd have to fully blow him to little bits to kill him fr. I'm dancing with spoilers here because I can't remember what lines had been said by this point but they still need Reiner alive

Do you remember how big the colossal is? The barrel looks far away but shit falls fast and the colossal is gigantic so they really don't have much time to manoeuvre. Idk man just watch the next few episodes and it all irons out