Absolutely correct. On a longboard at speed, you have to keep your weight forward. This is partly to keep the front truck under control, and partly because flex in the board means that the end of the board with most of the weight steers more.
If you panic and lean back, you're effectively only steering with the rear truck, which is not a dynamically-stable situation. Things go wrong very quickly at that point.
Ah, I tend to keep my weight fairly centered at speed so I'm stable if I have to break tuck. Also, I don't ride a deck with any flex, and I ride split angle trucks so I inherently have more turn in the front. But yeah, for beginners, weight on the front.
Good point. A centered stance might be ideal, and may just seem 'forward' in comparison to the typical shortboard stance.
And now I think about it, it might not be board flex that limits front-truck turning when leaning back - rather, the lack of weight on the front truck might allow the outside wheel to lift. Either way, it's a bad situation.
If you panic and lean back, you're effectively only steering with the rear truck, which is not a dynamically-stable situation. Things go wrong very quickly at that point.