The reason Noctis is sliding off Titan has nothing to do with the demo being "janky" it has to do with the way it handles the collision of Titan's arm. You're not able to scale or climb on top of enemies in this game, therefore, Noctis slides off of enemies instead. Plenty of games have done the same thing in the past, it's a very common practice to where I sort of recognize it on seeing it and relate it with other games or systems in my head.
The only thing "janky" about the E3 demo was the cuts it made for the sake of brevity and speeding along the battle. As for the overall demo itself, giving users multiple ways of defeating Titan would confuse the player. It's better to just hold their hand so that they can perform one scripted event they know has high impact and can show what they want to show.
Keep in mind, these demos tailored for E3 are just that... for E3. They're designed to get across a concise message and something that can be shown and repeated. They create a sandbox for you to explore where they make all the rules. It's much like watching someone demo a game on stage where everything happens for a reason, even if when you play the game it isn't exactly as scripted as it looks. That's because they want to give you a certain impression about the game to get you excited.
So, why have it so you parry Titan and then freeze his arm off for the fight? Because it looks cool, and it shows off the many aspects of the game and makes Noctis look like a huge badass. Why wait on all of your party members to show up? To introduce that you have multiple party members that will help you out in battle and show their support through actual battle actions or sometimes little events like when Gladiolus helps Noctis defend against Titan when you travel around the side of the area. Then you have all of the four party members throw Blizzaga at once to show their team work and ultimate take down of Titan with Noctis delivering the final blow to cut off his frozen arm.
Everything was done for a reason so that everyone could get the same reaction and the same epic feel. If you found a way to defeat Titan that just involved chipping away his health or something, it wouldn't really be that impactful, and not everyone would get the same reaction.
The point of Tataba clarifying later after the demo telling people that this is just a small slice of the possibilities allows the players/viewers to let their mind wander at the many possible scenarios that could play out in this small chunk of an epic fight and it gets them excited. Let the player imagine the possibilities, it's by far a better way to get people excited then to shove everything in their face at once.
Overall, the only thing that was negative about the Titan demo was Mat unable to dodge and failing at the MS conference, otherwise, it seems like a lot of people really enjoyed it, especially the music.