Most fantasy is post-apocalyptic in the sense that it typically involves a background of older, greater, more advanced societies that have collapsed. This has to do both with making magical macguffins rare and irreplaceable and a transposition of tropes related to medieval European views of the fall of the Roman Empire as well as more general myths and legends in premodern societies of lost golden ages. Scattered, post-collapse societies are also cheaper to film, can have simpler worldbuilding, and lend themselves to being populatated by a limited, manageable cast of characters. In a sense, there is nothing in this show radically more post-apocolyptic than the movie's visit to a ruined, troll-infested Tir Asleen. Obviously if we take this view to the extreme almost everything becomes "post-apocalyptic" and the term becomes useless, but I do think it is worth keeping in mind that fantasy typically has elements of a post-apocalyptic setting and ideas of collapsed, greater past societies are pretty thoroughly baked into the genre such that authors will incorporate them without necessarily always giving the matter much thought.
That said the show does seem to be introducing an element of lost advanced high tech or magitech to the setting with both the flamethrower and the way it portrayed the cuirass powerpack Boorman recovered being used in flashback, which certainly feels like a new element in the setting and a genre shift more towards the sci-fi. Perhaps their fantasy planet will turn out to be in the Star Wars galaxy for some sort of grand Lucas conjunction.
