Majora's Mask was a direct sequel to Ocarina of time and it was not the same world. Wind Waker was a direct sequel to Ocarina of Time as well that took place in the adult timine, and it took place on the great sea. Spirit Tracks was a direct sequel to Phantom Hourglass, which was a direct sequel to Wind Waker, and those all took place in different areas as well.
This would actually be the second direct sequel in Legend of Zelda history (a link between worlds being the first) to do this, so that is a pretty clear indication as to why people would wonder if it would take place in Hyrule or a neighboring region/reality.
You see Hyrule Castle in the trailer it was a given its the same Hyrule the will likely be changes made anyway.
Just because it showed Hyrule Castle in the trailer, that guarantees nothing by itself. It could have easily shifted to a neighboring land or a Termina type of scenario for the game itself, which seems to be what many people were thinking before this news.