It's a safe assumption, and definitely not faulty at all. It would be unreasonable and naive to expect anything else from them. Every single game in that series has been arcade mode only. They don't care about realism. We're passing judgement based on lies about historic accuracy the developers have used to advertise their games, and evidence provided by the same developers. Unless they completely change the game from the feel presented in the trailers, which we all know will not happen, I don't have high hopes.
Before I begin, I just want to say that I hold no hard feelings against you, personally -- simply the mindset you took to this thread. I don't mean any disrespect or personal offense by the following.
Realism has its place. This place is not in a game about two secretive factions who've maintained control over the world in one way or another since the beginning of organized human civilization fighting over the relics of a precursor race. An Aubrey-Maturin game, or similar, would call for a realism. But it'd be better to treat this game with a swashbuckling adventure mindset than a realistic tactical naval combat mindset. The plot unravels when you don't limit your brain to the confines of the universe.
And still, to be fair, there are no krakens, dragons, sea turtle islands, voodoo magic, or what-have-you. It's much more realistic than, if just as fantastic as, any Pirates of the Caribbean film.
Other than that, I do understand that you're also complaining about how they present the game. No argument there. It's not going to be near as realistic as they make it out to be. Still, I would not even dare to label the AC main campaign as "arcade mode." It deserves more respect than that. It's not you ascending levels of a tower or any other equally linear progression. When you punch people, they don't fly off the screen like God Hand, either. It's an open-world, free-roam game, and although it's set in something a bit like our reality, it's not going to be as realistic as you want a game to be. It's no Bad Dudes, it's no Tekken. It will be as realistic as it can possibly be while still being fun to the average AC fan who, most likely, hasn't even seen Master & Commander: The Far Side of the World, much less read the original Aubrey-Maturin novels. I wouldn't much like to play Cap'n Jack in an open-world game. A game adaptation of those novels, or any heavily-researched idea, would HAVE to be different than the kind of game AC4 is going to be. Like I said before, realism just isn't called for here. In excess, it would detract from the experience. So would krakens and whatever.
Anyway, you could just chock up any of the inaccuracies of the gameplay to being not the actual realities, but to the memories of Edward Kenway after his sailing life was far behind him. Granted, that's still a stretch.