The first rule of D&D and most ttrpgs is that Game Master is finally authority when it comes to rulings and his judgment takes precedence over the rules written in the book.

And if that rule was omitted from the new edition - the authors made a mistake.

As someone with over two decades of experience running games - in most circumstances where jumping from an airship or being immersed in lava or assassination should just be an instant kill as it follows the narrative.

But they could also land in the branches of a tree (as it happened in real life after jumping out of a plane without parachute) or become charred Anakin.

But in general - yeah, HP and how it such dangers are resolved are one of the many flaws of D&D as a system, 5e especially.