It actually does hint at mega starters. If you agree that
a) a demo is likely to display regular playthrough, that is, the storyline.
b) mega starters are guaranteed to be part of the storyline.
c) starters are confirmed to be involved in the demo.
Then you've got to admit that all the info we have hints that the megas in the demo are the mega starters. Sure, it's not confirmed, but if you had to bet which megas will be featured, then mega sceptile, blaziken and swampert are the safest bet. Now, this doesn't mean that even if the mega starters are in the demo, Blaziken will be getting a new mega evolution, it just seems to be a plausible possibility given the treatment fire starters and even Blaziken itself has got in XY. The mega starters could very well be there along with a new non-starter mega evolution. Either way what doesn't sound likely at all is us facing Wallace and mega Milotic, since Wallace only shows up late in the story, and the demo is likely to feature a part of the game where the starters are not fully evolved yet, that is, way before the last gym. But since you mentioned Steven and mega Claydol, I guess you were just joking.
That's a very good argument against a second blaziken mega evolution. On the other hand, Blazikenite was only obtainable during a short period of time, so the imbalance wouldn't be that huge. But I'd be just as glad if Exploud or Slaking (final evolutions of pokémon randomly showed on the same page) got megas instead - even though Slaking doesn't need a mega as much as it needs to get rid of Truant.