You are right. Arrive at the correct answer by multiplying the wrong value by another 1.5. Comes out to 830.4 tons of TNT. Which coincides with MichaelSmith's comment: "I saw another estimate of about a 1000 which is close enough."
If it happens once, it's a bug. If it happens twice, it's a feature. If it happens more than twice, it's a design philosophy.