Try this: Take the same picture from the same spot with each camera and the 70-200mm lens + 2X extender. You will see that the angle of view is narrower for the APS-C camera than the Full Frame camera. If both images are then displayed at the same size, the image from the APS-C camera will have been magnified to a greater degree than the image from the FF camera. That is what the article you linked to in your question is referring to: images from cameras with smaller sensors must be magnified to a greater degree than images from a larger sensor to reach the same display size.

Another way to look at it: take the image from the 5D mark III and crop it so that only the part that was captured by the center 40% of the sensor (that equates to an APS-C sized sensor). Then display it at the same size as the full image. You will see that it looks like the cropped print was zoomed in by a factor of 1.6x. What you give up in the case of cropping the same image is resolution. The original file is 21.1MP, the cropped image is about 8.24MP. That is lower resolution than your 12MP APS-C 450D.