There are far more factors determining wrist position than the size of the keyboard

Ergonomic keyboards are not a result of "the size of the keyboard", but the shape. The size could be identical, it is the shape that matters.

Without any real studies on it mentioned so far you're relying on gut feeling and logic here. Well, you mention sitting with proper posture actually helps, which is putting your body into proper alignment. That makes sense, if your neck is arched and your back is crunched all day it will eventually cause damage to your discs and cause nerve pain.

Why doesn't the same apply to your wrists? It seems logical that keeping your wrists cockeyed all day would put strain on them, and that keeping them in alignment would reduce strain.

At the very least it seems easy to see why some people would genuinely prefer keyboards like that just for comfort. I find it hard to label as "snake oil"