Really? That's interesting. I thought the King of Sardinia was the titular (although I suppose a claim does not make a de jure reality) King of Corsica?
Storytime!
Corsica and Sardinia were originally claimed by the Pope in the 11th century on the basis of the fraudulent “Donation of Constantine,” which purported to show that Constantine had donated lands of the Western Empire, including “the various islands,” to the papacy. The Pope never ruled the islands directly, but did at various times grant ecclesiastical control over them to the Archbishops of Pisa and Genoa.
In 1297, Pope Boniface VIII created the “Kingdom of Sardinia and Corsica” and granted it to James II of Aragon. James did not actually control any part of Corsica or Sardinia at the time, but Boniface gave him title to the islands in order to purchase his help removing his brother Frederick from the Kingdom of Sicily. James never actually got around to doing that, but he kept the title, and in 1324 made good on it by invading Sardinia. The conquest of Sardinia, however, took nearly a century to complete, and in that time Aragon made no attempt at Corsica. Only in the first half of the 15th century did Aragon actually try to wrest Corsica from Genoa (which had fully acquired it in 1347 after defeating the Pisans), but despite occasional success they were never able to permanently drive out the Genoese and eventually gave up. Other countries tried at various times in the 15th and 16th centuries, most notably Milan and France, but the isle remained Genoese.
Thus, while Sardinia remained under the control of the Crown of Aragon, Corsica did not, and this
de facto split between the two islands eventually led to the split of the title, apparently in or around the reign of Ferdinand II of Aragon. In the early 16th century the Crown of Aragon became part of united Spain, and the Spanish monarchs styled themselves "King of Sardinia" and "King of Corsica," with the former being actual and the latter purely in pretense. They never stopped using it: “King of Corsica” is, in fact,
still one of the titles used by the King of Spain in 2017.
When the Kingdom of Sardinia was formally ceded to Victor Amadeus II of Savoy in exchange for Sicily in 1720, it was very clearly
just the Kingdom of Sardinia, as the old medieval composite kingdom of “Corsica and Sardinia” had been obsolete for centuries. As far as I know, the Savoyard kings never laid claim to Corsica and never styled themselves as Kings of Corsica, even in pretense.
The first Doge of Genoa to also be crowned King of Corsica was Giovanni Francesco Brignole Sale in 1637. This did not reflect an actual change in the status or ownership of Corsica; rather, the lapsed title was “revived” as a way to bolster the independence and stature of the Republic by elevating the Doge from a mere duke to a king, thus making him equal in principle to any other European monarch. While the Kings of Spain were still using the title “King of Corsica," this was understood to be a title in pretense only, and as far as I know no objection was raised in Madrid to Genoa using the title in fact. Thus, by the time Theodore arrived on Corsica, the Doges of Genoa had been crowning themselves kings of Corsica for 99 years.
In other words, Corsica was a well-established royal title with a medieval pedigree just as old as that of Sardinia, and which had been continually claimed since its creation by Aragon, Spain, and then Genoa (even if those claims had not always been exercised). One could dispute Theodore’s legitimacy as King of Corsica, but nobody could reasonably dispute that such a kingdom and title existed.
Theodore knew this history very well, and in fact wrote letters to the pope urging him to renew the papacy’s ancient claim to the island. He proposed that he would conquer the island from the Genoese in the pope's name, acknowledge Corsica as a papal fief, and rule as the pontiff's vassal king. All he wanted in return was pontifical recognition of his title (and, if possible, some monetary support), which even with the rather slight temporal power of the 18th century papacy would have been helpful in legitimating his rule. The pope, however, never gave him a reply.
Considering Theodore’s rather dubious commitment to Catholic orthodoxy, this might have made for an interesting relationship.