Nine miles does seem like an exaggeration, but it could easily be 8 to 8.5 miles to play all the back tees. Consider:
Playing length, as you say, is 5 miles.
If the green-to-tee walks are each 100 yards, that's 1700 yards, or one more mile.
Many of the walks to the back tees are quite long walk-backs, I'd guess that is at least one more mile for the total.
So we're up to 7 miles, and we haven't included anything to get to the clubhouse and back, or accounted for the fact that nobody hits the ball down the sprinkler line and walks the shortest distance the whole way round.