Castex's line about showing that "There is more damage in leaving the union than to remain" was a tail-end comment in his letter. As one French news website put it, the effect was that "Lord Frost loses it in a Twitter tirade".
Castex's main thrust was that the UK should respect the agreements it's signed. Never mind Frost suggesting a dispute settlement, Castex suggested weeks ago that if the UK doesn't grant the licences, "we will make use of the arbitration panel of the agreement to lead the British to respect their word and we will question all the conditions for the more comprehensive implementation of the agreements concluded under the aegis of the European Union".
Whoever leaked it, the letter was "interpreted" by Alex Wickham, 6 years making up stuff at Guido Fawkes and setting honey traps for Tory MPs. "France tells Brussels it must demonstrate that Britain has been damaged by leaving the EU" isn't quite what Castex said, but I don't think farmers and fishers need it to be demonstrated anyway, now they see the reality.