There are two kinds of 'pinch' flats.
Most folks call a flat caused by a under-inflated tire (relative to rider weight and riding style) going over a sharp edge/rock, causing the tube to be squeeze between rim and road object cutting the tube usually in two places (like a snakebite), a pinch flat. These occur with correctly installed tubes.
The other kind being discussed here is where the tube is improperly installed and gets part of itself squeezed between the rim and the tire or if the tube is twisted/kinked in the tire causing it to squeeze against itself when inflated. These may blow during the very first inflation or may last a few minutes to hours and in some cases can even be used before suddenly blowing with no specific event just prior to blowing. I don't usually hear these called a pinch flat.
Al