There are a bunch of arguable reasons for the tuned up fight and they're all pretty interesting. They really don't want you to win, but to fight him, lose, run out of tries, and then say "See you in the real game, Magnamalo, where we fight on my terms!". As a whole, as many many many people have said over and over, the demos are never representative of difficulty, etc, as they intentionally give you weak armor but strong weapons. It's just done as a way to build excitement for fighting with actual progression. It's a very interesting strategy, and it's primarily thanks to the sheer size of the audience - the vast majority of demo-players are returnees, so they 'can't' just create easy fights exclusively. It's why they've always offed 3 tiers; beginner friendly, challenging, and very challenging.
You can also tell they really don't want the average player to win with the clamped time limit forcing sheering aggression, which is almost certainly going to get players killed. Normally you'd have 50 minutes, not 15; at that pace, we could all certainly handle it slowly, taking our time to fight on our terms. That's what the full game'll be like!