I love this because this wasn't even real life, it all happened in Sherlock's head, and the reason Lestrade in Sherlock's head helped is because Sherlock looks at Lestrade as someone who mostly follows and listen to everything he says.
Sherlock expects Lestrade to help because he is Lestrade. Basically in Sherlock's head Lestrade is his own little minion that will do anything for him...