which are something like counter-sites, a kind of effectively enacted utopiawhich are something like counter-sites, a kind of effectively enacted utop

Again, antithesis shows here; just as heterotopias are a counter to utopias, heterotopias themselves are still counter to the reality of Society. They are a physical manifestation of the utopia, showing yet another implication of the antithesis: the inherent sameness of both items being contrasted. The heterotopia is merely the utopia realized, so while completely different in its realness, it is still the same.

On the other hand, it seems Foucault is having some second thoughts at the antithesis nature of the heterotopia and the utopia. “Something like” and “kind of” are usually throwaways, and any who value style will remember to just get rid of them. But Foucault keeps these throwaways, which suggests the sameness between the utopia and the heterotopia is not complete, and thus the pair do not constitute a full antithesis. However, it is important to keep in mind that, once again, utopias are not real, nor can they be. So an “effectively enacted utopia” can only ever be a “kind of effectively enacted utopia;” otherwise, it will cease to exist.