If the train was that packed, you could always go BACK to King's Cross and get the train from there.
But, can I also ask the obvious question; besides it being all stations (I know, I do it every day from Old Street) why would you get off a train that is already going to Hatfield to get another train that goes to Hatfield? Yes, the other train is slightly quicker - but not by much considering the wait at Finsbury Park that lets the slow get a few stations ahead. Without checking, I'd hazard a guess that there's maybe 5 minutes in it at most.
Okay, he might have got on a Hertford North train but why didn't he just wait for the right train? At Moorgate, he'd be getting on at the starting station and could have any seat he wanted! Yes, it gets quite crowded by Finsbury Park, but most people will have alighted at New Southgate, Oakleigh Park or New Barnet. From then on, you'll usually be the only one on the train. If I go to WGC, maybe 5 or 10 people get off the WHOLE train!!
Using the Great Eastern (sic) route would have been a good idea before, and I am not sure why the author thinks that it is some magical idea. Anyone with an ounce of common sense would have used it if the TL side was down, just as people use the TL side when the GN side goes down. FCC even runs buses between the two branches in times of serious disruption, or you have local buses.
If you're REALLY in a rush, there are ample taxis available too.