The point of short/medium/long cage size in a derailleur is how much slack it can take up. This is generally measured in terms of difference in teeth. For bikes with multiple chainrings up front, and particularly ones with big variance in size, all this matters. But the huge majority of folders have a single chainring up front, so the only difference in teeth is the rear cassette, and that's essentially always well within the range of even a very short derailleur. For example, a ginormous 11-36 rear cassette has a 26-tooth difference. But the SRAM X9 short has a conservative capacity of 30 teeth.
Frankly I don't know why folders don't always come with short cage derailleurs; it'd raise the chain further from the ground and on a folder that is always a huge plus.