Sorry lads/lasses, I don't think DJC aerials are in this game.
Whole thing appears to be based on a misunderstanding by Sinji:
https://twitter.com/Sinji93/status/1008861138891788288
Scroll up in this Twitter thread and it appears that Sinji thought he saw Mewtwo do a djc FAir, but in the Tweet linked, he corrects himself with what he actually saw - cancelling Shadow Ball's charge with a double jump. This is a new mechanic to this game so it's not unreasonable for somebody to see this and confuse the two.
You may also notice that the original Tweet that this thread was a response to has since been deleted.
The thing about having to input DJ + aerial in a specific frame window to get a DJC also appears to be based on something Sinji said:
https://twitter.com/Sinji93/status/1008850565634428928
Therefore, I feel we must remain sceptical of this too.
Considering that nobody else has reported nor recorded DJC from the demo despite there being two demo characters that can allegedly use it (Ness and Mewtwo) and I think that until clear evidence surfaces to the contrary, we have to assume that the whole thing is a misconception and there are no DJC aerials in Ultimate.
Not gonna lie, this is a bit of a kick in the balls - I briefly got excited about how much better Ness' neutral would have been with DJC and things like the new nair he has and other cool applications of DJC. Would also have been a nice way to freshen up the character as well and make him so much deeper than he ever could be without it.
Ah well.