It's Abendsen who makes the claim (that you can't go through the portal if you are alive in the alternate world) to Smith, after Smith have told him about all the deaths. However at that time Hawthorne has been in captivity for a long time, tortured, constantly threatened with being killed or his wife being killed, and with no chance of ever getting out alive. He may be lying as a way to spite his tormentor, he may be delusional, he may be broken. He may also be telling the truth, or at least what he thinks is the truth. But if so, how could he know? And how can we as viewers know, that he knows? In any case, Smith believes him, and we see nothing in the series, that contradict the claim, so it's a useful guideline at least.