Theologians would want to disagree, but I would argue that an overwhelming number of scientists does constitute a legitimate authority on whether or not something (god or otherwise) does or does not exist.
Example: Scientists would want to disagree, but I would argue that the overwhelming number of people in the world who believe in a god (of some form) does constitute a legitimate authority on whether or not something (god or otherwise) does or does not exist.
See how that works?
But my claim is that a consensus of scientists is an authority on matters of fact of the natural world, specifically. In the same way that a consensus of programmers would constitute an authority on programming. So that it's fair for Dawkins, who views the (non)?existence of god as a fact of the natural world, to appeal to scientists as an authority.