a STO has to go through regulations exactly as a IPO does, a STO has to be traded on a ATS or through a broker thus not being allowed to freely transact in a currency like manner thus not making it a cryptocurrency.
To the best of my understanding, STO is another form of token or cryptocurrency offering that is regulated. It is still a cryptocurrency and I'd maim difference from what we used to know as cryptocurrency offering ICO, is the security.