He didn't say that truth was relative, he said that our understanding of truth was relative. We see it from the point of view of those who live here, in this moment of time and thereby are subject to the same things which make the Creation appear to be older than what it is. God, who is outside of time, left a record that says it was only a little less than 6,000 years ago that he did all this.
1 God year = 750,000 human years. Easy! But as we had to invent the concept of zero at one point I can see how our Creator might want to dumb the math down for us a bit.