If you watch fencing competitions, you can see that people get "pushed back" by attacks simply in their attempts to avoid them.
Only because you can't step sideways in fencing.
In reality, you do not have to throw people over a ledge, because opponents do sometimes step backwards, lose their footing, or even, yes, get pushed back from a blow. As someone who has been on both sides of "pushback" I can assure you that, while it doesn't work exactly like the old Hollywood "fly backwards from a hit", it does occur, even when you just run up, hit them and then stop.
I agree, that if somebody sat right on the edge of a cliff, they could topple from a shove...but that's not what is taking place in the module.
Physics says yes. It is called "transfer of momentum", and you can see it any time you wish on a pool table. You can also grab some practice gear and a friend, put a cord down to indicate the ledge, and see if you can get each other to go past it. For more fun, actually use a stage about an inch or two over some soft surface where you won't get hurt if you fall.
If you were fighting an inanimate, spherical object, you'd have a point...but in this case we're talking about a living thing, who is not standing directly on the edge, but rather a relatively safe distance back from it, and can change his center of gravity, and has arms to grab with.
Transfer of momentum also takes into account the other laws of motion, like the opposing force, and gravity.
One thing you will notice is that, where the fall is not due to missed footing, or backing up to avoid being hit, transfer of momentum tends to be partial. That is; if one falls, you often both fall. And that is the difference between the DCC warrior and you - the DCC warrior is better at pushing, at stopping, and at intimidating his foe to take the fatal backwards step. No magic required.
My mileage does vary because I have watched sumo wrestling, which is what we're talking about. And it's NEVER a simple matter of simply slamming into somebody. If it were, there would not be a sport.