It is true. 3D movies are really not 3D at all because we can't see in 3D in the first place. Let me explain why.
We Only See In 2D.
Cube
We human beings and every other animal that has the ability to see can only see in 2D. This is because we live in a 3D world. The image we see with our eyes is 2D, like a picture, even if you are aware of the third dimension. If we could really see in 3D, then we would be able to see all points of a 3D object at once. just as we can see all points of a 2D object at once. We can't see all points of a 3D object at once just as 2D creatures can't see all points of a 2D object at once. If you look at the cube on the right, you might think you are seeing in 3-Dimensions, but really your just seeing three rhombuses, one red, one blue, and one green. They are put together to look like a cube. the reason why the we see in 2D is because we can't tell if objects are 3D unless we pick them up and rotate them to view all sides.
3D Movies Are 2D
Stereoscopic Image
We can only draw in 2D. We can draw any 2D object we want on 2D surfaces but we cannot draw true 3D shapes that can be rotated to view all sides. Therefore, any movie that you see is really 2D. The theaters are really making movies that allow you to have better depth perception. The term 3D in movies came from the idea of things popping out of the screen. However, just because it comes out of the screen, doesn't mean that it is real or 3D. You are still seeing a 2D image of an object that's getting closer to you. The cool depth effect is achieved with something called Stereoscopic 3D. The projector that displays the movie on the screen is actually projecting two images at once. that is why when you look at it normally, it looks blurry. The funny glasses that you wear when you go to a movie only allow your right eye to see the right image and your left eye to see the left image. The two images are slightly different. Your brain combines the two images your eyes catch just like your brain does normally to everything you see in your life and give you a better depth perception in the movie. It looks like the screen is a window to the world the movie takes place in and that you could step right into. The image above shows two slightly different images that if seen in Stereoscopic 3D, would look like the rectangle is floating in front of the red background, all because the placement of the rectangle is slightly different. However, Stereoscopic 3D is not really 3D since we can't see all sides of a 3D object on the screen at once.
Drawings Of 3D Objects
Cube Drawing
Like I said before, we can only draw 2D shapes on 2D surfaces and we don't have the ability to draw true 3D objects which can be rotated to view all sides. So you might be asking about the picture on the right. You can't take this cube even if it were printed on paper and rotate it, therefore it is not a 3D drawing. It is really 2 Triangles, 4 Trapezoids, and a Rectangle put together to look like a cube. Another reason why it isn't a 3D drawing is because the angles are not right angles. It is a cube, and cubes are supposed to have right angles on all corners just as squares do. However, since it is a 2D drawing of a 3D object, the penalty of losing a dimension is that it loses its right angles.
Well there you have it. It might take a while to understand, but we really do see in 2D. However, don't feel bad, we aren't the only ones handicapped. 4D creatures can only see in 3D, 2D creatures can only see in 1D, and 1D creatures can only see in 0D, and 0D creatures can't see at all. And just because that we can't see in 3D, doesn't mean the world doesn't have 3-Dimensions. We can easily figure out if an object is 3D by picking it up and rotating it to view all sides. Just because we can't see the other side of a building without walking to the other side of it doesn't mean it isn't there when you're not looking at it. We've been around 3D objects our entire lives, so we don't even bother to check if they're 3D, we know through sight recognition. So be thankful you live in a 3-Dimensional world which includes up/down, left/right, forward/backward, and even a fourth dimension called time. Enjoy life while you're here, and hope that we awaken in a 4D after we die.