blah blah blah. I will set fruit aside.
You can eat as many vegetables as you want with lean meats and not get fat (and no potatoes don't count).
Simply and easy and almost certainly true.
And yes while fiber is a large component, so are other things like water content all cumulating in a very very low caloric density.
Its damn hard to get many calories even if you ate 4 lbs of green beans or lettuce. You might be able to argue that some of the starchier vegetables are too starchy yada yada, but with a basket full of various vegetables, and meats, lay people will be hard pressed to feel hungry, not get enough fiber, not have important micro nutrients, etc. etc. and as such unlimmitted vegetables atleast works as a good rule of thumb.
Issues can of course come up though if your vege's are swimming in ranch dressing.