Naming Napoleon a 'usurper' is absolute nonsense and falls into line with British, Bourbon, and allied propaganda of the period.
Of course he was a usurper in the eyes of the Bourbons. The rightful king of France having been deposed and his successor done away with, Bonaparte, a commoner and barely French, had subsequently assumed the crown.
Having run his course, Bonaparte eventually abdicated and was sent into exile but then returned from exile to resume power, joined by mutinous soldiers From the monarchy's point of view, and indeed that of the powers gathered in Vienna, that was an act of insurrection.
At that point, your assertion that France was not at war becomes moot