Hitchens seems to be working off his assumption that a decision not to run connotes an acknowledgement of failure/weakness on his part:

Should he make up his mind not to run, he would retrospectively abolish all the credit he has acquired so far. It would mean in effect that he never had the stuff to do the job and that those who worked and voted for him were wasting their time. Given his age and his stature, can he really want that to be the conclusion that history draws?

Why such a decision should connote anything except principle, pragmatism, or simply preference is beyond me.

That's exactly what I thought. I have no idea what Hitchens was trying to say with that. I believe he was just trying to spice his article with a bit of controversy in an otherwise unusually bland piece.

His piece was non-news opinion about what Gore might do if he might happen to win the peace prize. Not much for Slate to lead with. Still, it could have been interesting, if the reader were not distracted by Hitchens' unsupported tangential assertions and exagerations and his non sequitur conclusion, stated as if it were self-evident.

I saw that as more of an assertion which, coming from Hitchens, is meant to be taken as a fact since the rest of us can't possibly be anywhere near as knowledgeable as he believes himself to be. A premise would be something that lesser mortals such as ourselves would create.