It was dealt with quickly and the train left under its own steam (!).
There are actually quite a few reports of fire/smoke at Moorgate, and sometimes I wonder if it's down to a spark from the third rail lighting rubbish, or someone thinking they saw smoke which was actually someone vaping at the end of the platform. Many reports of smoke turn out to be false alarms, which gives credence to my theory.
If you saw it yourself you'd know it wasn't the latter.
It's usually down to oil/rubbish sparking. Flames aren't common, usually just smouldering and a lot of smoke.