He was. Artur even pumped out a cringeworthy youtube video about it, despite literally all the evidence suggesting it was Kanjuro (yes, I've also supported Kanjuro = traitor from the beginning, it was just so obvious).
Heck, he tried to defend it against all logic until the bitter end, despite everything pointing in a different direction.
But that's Artur. Once he has some silly idea, he really believes this has to be the one and only truth.
Same with Carrot being a future nakama. Mind you, that could still happen, but Artur just ignores all discussions about it. For him it's a given.
Same with Artur labeling Yamato a trans-man (lmfao) and truly believing this is a trans-matter, despite everyone with some common sense seeing that it is not ...