It was the 2.5 launch event that was held at Disney Animation studios. It was actually remarkably similar to this event in that there was trailers, live music performances, etc. (Though the launch event had a Q&A session with Nomura). And as for that trailer, you're right it was never released to the public but I wouldn't say all of it's contents were shown in the E3 trailer. My memory may be a bit fuzzy (It has been a year since I was at the event) but what that trailer showed was Sora walking around the room that we've seen Xehanort/Eraqus playing chess, a panning shot of a city resembling Daybreak Town, and some incomplete Heartless animations and Keyblade transformations (was shown only on a white background). Obviously the Chess Room and Keyblade transformation were later shown off in that E3 trailer though.Maybe it was the 2.5 launch event?
But I recall there was some event at Disney Land for a Kingdom Hearts 3 trailer, no trailer was released to the public and judging by the information from whom attended that they described the contents of the trailer that was shown at E3 later that year.
(Eraqus and Xehanort playing chess in a room. Sora's keyblade changing shape, Tangled world, and so on.) That information was shared at Kingdom Hearts insider.
Honestly, that footage was so rough and incomplete that I'm not surprise they never released it to the public.