The difference lies in the experience of the sexual attraction to people. A person who experiences sexual attraction can feel that towards a beautiful stranger, but not desiring to act on it due to "only knowing them for an hour". That person would still have those feelings, though. A demisexual cannot experience sexual attraction to anyone, until that deep emotional connection is formed. There are no desires/sexual feelings to act on, when meeting people.
In short: it's the ability to feel sexual attraction (biological), not the desire to act on it (preference).