What is your definition of a "map"? I understand that going inside a building and loading another room/map would be cheating, but what about the Super Mario Sunshine method, where the room is far away and it just teleports you? (explanation here: https://www.youtube.com/watch?v=YmYajM_vBMA)
After watching the video, I do see a few issues. This may disqualify a game for many reasons. There are concepts of these 'invisible rooms' that have my concern. The video states that these areas are just redrawn. This language shows that it is indeed a new map. These 'invisible rooms' are located above the current map. 2D games typical work with only two dimensions, most being height and width/length, leaving out depth. I have made top view 2D games that use length and width before, leaving out height. The glitch incorporates many 3D aspects. Unfortunately, you used a 3D game to try to describe a concept to be used in a 2D game. If there is a different example, that explains it different, I may accept it. I have also made maps that had mini maps on the same map. In my mind, it is a way to put many maps on a single map. They are still many maps. I will have to look at this. I wish it was before the Game Jam started. Rules could have been clarified or modified.
I do understand what you are talking about. Most 2D games do actually use 2.5D graphics. They show the traditional height and width, and attempt to represent the third dimension depth. Almost all RPG Maker games have this 2.5D imaging system.
In my personal games, I like to eliminate this .5 dimension, and make what I call 'Tru 2D' images, using only height and width/length, with no depth representation at all. This also means no shadows. I have made one true top view 2D game where only length and width were represented, with no height.
Answering your question, yes 2.5 images are allowed since they attempt to represent the 3rd dimension but actually are not 3D images. If you could send me a sample image to firstname.lastname@example.org, I could easily verify if we are talking about the same image style.