Isn't there a locking tab on the front sprocket? It has been awhile since I last changed out my chain & sprocket but most bikes have the tab to keep the counter shaft sprocket from getting loose.
Figure that the dealer would give the chain replacement warranty work to the noob mech.
I had replaced my chain about 10k miles before the recall with a DID Xring.
Good judgement comes from experience. Experience comes from bad judgement.