He has nothing to back it up - it's all an empty-city act.
Tips
history
From 《三国演义》: with the city of Xicheng undefended and Sima Yi's army approaching, Zhuge Liang throws the gates open and sits calmly on the wall playing the zither. Sima Yi, suspecting an ambush, retreats. The perfect bluff has carried the name ever since.
usage
Often 唱空城计 ('to play the empty-city stratagem') - to fake strength you don't have.