Company Procedures
I don't know what other airlines do, but on our airline's "Before Start" checklist there is a challenge "Gear Pins and Covers" with the response from the Captain being "Three on Board", where he has to physically see them as being stowed, (A300, A310). I don't know the type of aircraft that was used so they may have different procedures.
Was this done? I don't know. Did the gound crew have another pin inserted for the nose wheel for push back? It could not have been a towbar pin as those are welded to a chain that is welded to the towbar. Accurate information is needed before finger pointing starts.