The True and the False

Bertrand Russell says this in his Preface to The Principles of Mathematics (1903):

"what is true or false is not in general mental"

This is true to a point. The proposition "the Cambrian Explosion occurred roughly 543 billion years ago" was as true back then as it is today when us late comers on the scene found out that this was a true statement. But then how does a materialist square this with his views? How do purely physical entities have truth values? Its true that human mental states don't make propositions true or false but they still seem like irreducibly mental categories to me.

So how does one assert the aforementioned proposition as true without positing a Super mental basis to the universe and the way it was created? (I believe the later Fregean philosopher Michael Dummett proposed an argument for God's existence in this way in his Gifford Lectures).