I'm afraid that isn't a terribly compelling line of reason. A better one would be "Yes, conjecture is part of the scientific process. It is ordinarily followed by the design of an experiment to test the conjecture, otherwise known as a hypothesis." The fact that many archaeological and geological hypotheses are difficult or even impossible to test does not render them prima facie unscientific.
It may, however, leave them unsubstantiated, a point often downplayed by many in the scientific community who feel acutely the undeniable connection between their career track and their credibility.
Actually, this is the same line of "reasoning" used by creationists and IDiots. They spend all their time attacking evolution "thinking" that if they "disprove evolution", then, obviously, their god is the right one.
A false dichotomy if ever there was one.
The IDiots at Discovery Institute are notorious for this. And for speaking out of both sides of their mouths.