The "giant squid was considered a myth" thing is much like the "the earth was considered flat" thing; both are largely myths themselves. Nobody who has put any research or thought into the matter has doubted the existence of giant squids for a very long time. They have been scientifically studied for centuries, about as long as we've been scientifically studying anything. Before then, fishermen were finding specimens all the time.

The only thing that is actually doubted is nonsense like the existence of squid the size of ships, big enough to take on Naval vessels. Much like the round nature of the Earth was never in question, just the idea that it was small enough to make sailing right around it a sensible thing to do.