Sam did not know that he guessed it right. Jack has already activated a hidden artifact that will be releasing the poisonous gas in that room, and using that chance, he and everyone will escape from that place.
One thing everybody understands is that they would not be able to win against Sam as he is more powerful than them.
Even both of his reinforcements would not be able to protect him or Oron. Other than that, he did not have any other artifacts to protect his life from Sam.
Also, if he wanted to wait for the main backer, then it would be too late for him. Because of this, Jack activates the hidden artifact, which already begins to release the poisonous gas.
But the thing is that there is a chance that he also got affected by the poisonous gas.
Because of this, immediately he signals everyone to take the antidote that he already gave everyone. But Jack did not realize that Sam also noticed his sudden movement.
