Wrong, a GOOD game demo gives you snippets of the story in order to make sure you're invested in the finished game, usually starting you off at the starting area of the game, that way they have an excuse for why the characters are both low levelled and ill equipped. By throwing us headfirst into a huge gameplay environment where enemies can ambush you if you take too long to beat them with no context as to why they are here, you run the risk of alienating the players. The Duscae demo only sets up why the players are here (their car broke down) and some mentions of the empire. And the area is clearly meant for a bit later in the game, so the enemy attack rates are a bit more elaborate than what a starting field would give out. This has been a common criticism about the gameplay, your characters are much weaker in comparison to the enemies of the game at the point it is meant to take place in. Considering the fact that you fight more effectively once you're past level 15, it's probably a good bet that it was originally meant to be played then rather than starting at the level we do.
Yes, a good game demo can demonstrate why you should buy it when it only shares a bit about the story as we know it. The Final Fantasy VIII demo, while it threw us into the invasion of Dollet without little explanation, offered enough about the game to entice players who played it to buy the full version. However, with a game that has been in development for as long as XV has and if it has indeed changed, it is a good idea to reintroduce the key players of the game via the demo. Having you feel like you've wasted time instead of done something doesn't make it any fun. This is also a double slap in the face because all of the stuff you do is tossed aside by Cindy fixing the car for free after you beat the Behemoth or do something else to raise the money. So basically we played through this demo and achieved nothing from it, which isn't the best way to make players interested.
And yes, this extends to marketing. The marketing has only focused so far on the Duscae region and that's it. We don't have any idea of what Luna's role is at this time, thus one of the big criticisms of her inclusion in the game. People say we didn't know enough about Stella, but we did know what her role was from the beginning and what her dynamic with Noctis would be. Showing Luna instead of Cindy would have been a good way to alleviate the eventual criticism of her not being as good as Stella might have been. Presentation is important and that's doubly true of your demo. You want people to be invested in the game and wasting your time in an area that's beautiful but is pretty irrelevant is not the best way to do so. This goes with my point that apparently has been missed, that the game developers are focusing too much on gameplay but refuse to actually tell us about the story and how it is different and why these changes were necessary. The marketing has failed to do its job as well as the demo itself. I have seen just as many people call off any interest in the game as I have people who got more hyped about it.
And no, my point is not about telling the full story, that's for the game itself. My point was starting us off in an area that introduced the characters we'd play as and show us why we should care enough to play the full game would have been better. Playing the game, I literally learned nothing more that would give me an incentive to keep playing and it is important you give people that incentive. That's what good demos do. Duscae does not and infact has turned off quite a few people because they found it both confusing and boring. The characters aren't incredibly interesting in this either.
And one thing I do want to add, is that this is fair criticism for any demo that relies on feedback. Yes, it is a fully gameplay video with little story to it, but wouldn't you want your feedback to also depend more than just on the gameplay? And that's what Episode Duscae is. It's a three hour gaming test bed instead of a demo that's meant to have us be interested in the game. It was time and energy that should have been spent working on the game itself and the marketing for it. It is, as many have pointed out, completely pointless in the grand scheme of things. And you don't want that in a demo. You want something that people want to keep playing as they wait for the finished game or just want to try it out and see if they like it. Besides, as another person pointed out, test players are already hired to test games and give feedback about the gameplay before they are released.