I would argue it's not a proper market place, fantasy or not, without such ability. (meaning, whatever values or data that comes from this is of near zero utility without the ability to short)
I'd guess the comment was less about 'what people think is proper,' and more about the fact that there are typically no mechanisms by which to short very early stage private companies.
last time i checked, every single fantasy football site also didn't allow you to borrow money from money sharks to try to bet against on a bad line up.