It is a combionation of more factors, but you got it pretty close. A VDG has very little energy stored in it, so the length of the pulse is very small, so even though the peak current could be high it last for a very short time, so it is not too dangerous.
A 50hz 400kV line is a different beast - the energy it can deliver is practically unlimited, so the only thing that limits the current is the resistance of your body. And with the numbers being what they are there could be tens or hundreds of amps flowing through you until your're burnt to a crisp, and the distribution site wouldn't even notice something is happening.
In a picture tube the capacitance is much higher, so the energy stored will be higher so the pulse would last longer which is another factor that would most likely kill you.