Eliacube might be able to reverse the time to 200 years ago, but
It has a small energy(wakfu) efficiency, so a lot of wakfu is lost when Nox is trying to reverse the time. Nox's technology and Nox himself are much insufficient to utilize Eliacube, so it causes only wastage.
Let's use an example to explain. Think of a circuit with a battery, a electric-conductive wire and a lamp. Now, Eliacube is the battery, Nox himself is the wire, the history reversing is the lamp(going to light up). Here comes a problem: this battery is too strong, yet the wire is pretty much weak. 100% output from the battery(Eliacube), some of the power lost in transferring via the wire(Nox), some of the power lost in reversing the history(to return the wakfu to the source, mentioned below). In this case, maybe the real power efficiency is lower than 5%.
Nox is reversing the history, so the wakfu taken will also be reversed and go back to the source, and thus it is "depleted".
So Nox is right when he says that all the destruction he has done will be reversed: ironically enough, even the wakfu he has taken also returns to the source as the result of reversing the timeflow.
Sadlygrove's death on episode 25 is true and he isn't supposed to come back alive, but then Nox, after his victory against Yugo, reverses the time for 20 minutes: Just enough for Sadlygrove to disappear again, but this time, the history changes a little and thus he ends up getting 'saved' by Rubilax.
In season 2 episode 5, a starving Yugo, Ruel, and Sadlygrove encounter, and subsequently devour, a small winged pig in the wild. Later in the episode, Yugo, Amalia, and Eva get turned into small, winged pigs by a trap in a dungeon. Makes you wonder where that first pig came from — and the end credits in particular seems eager to remind us this.