Well, he didn't mention gravity by name but, if you reread the first post, what he describes sounds exactly like acceleration from gravity to me.
The first post of this thread? I don't think there's any hint on gravity
EDIT: Increasing the vertical velocity component by a constant number every frame is precisely what gravity does..., isn't it?
You are right, it is. It represents some force (acceleration) pushing the object down and if you change the vertical velocity each frame by delta_time * gravity, then it represents gravitation force (gravity = 9.81on Earth).
The problem is that we really don't know what kind of behaviour is PiCroft trying to get. Because keeping the horizontal velocity intact and increasing the vertical by some number every frame would simulate something like a gravity or other similar force (if the acc value would differ from gravity), which would be like a free fall. Like an airplane flying forward and suddently losing its wings (with acc = gravity) or suddenly being pushed down/up by some magnet or something or just losing part of the wings lift (with acc != gravity).
Not exactly what I personaly imagined by "diving" and "climbing". That sounds to me rather like if the object (airplane? submarine? spaceship?) tilts up/down and flies that direction. Yes, maybe with the added grafity acceleration, I don't say he certainly doesn't want to have gravity. But when something flies straight at a constant velocity propelled by some engine (needed against air/water/xxx drag) and then tilts down to descend, you must tilt also the propelling force.
Doing it the other way isn't WRONG, it just isn't what I GUESSED PiCroft was trying to achieve
This really is complicated to get right when we don't know what are we doing