Two things to say here:

(1) The view articulated in that answer, that the Second Law only applies to systems that are genuinely closed, would render the Law empirically useless. There are no systems of this sort, except for the entire universe. But we appeal to the Second Law all the time to account for the time-directedness of systems that aren't completely closed (such as ice melting in a glass of water, or gas spreading through a room). We're really working with an approximate sense of closure, one that allows us to describe reasonably insulated systems ... (read more)