a 20"x20" image to be printed at 300 dpi would be 6000 pixels x 6000 pixels. what rendo is doing is advising you that if you submit an image its pixels will be divided by 300 by the printer and that will determine the printed size of your image. They just don't want you to be surprised that the image filling your monitor will be less than a third of that size when printed.

for example, you have your proverbial 1600x1200 pixel image (my monitor size). the image fills my 20" screen at its resolution. However, when printed using 4 colour offset at 300 dpi the image will be 5.3 inches x 4 inches in physical size. Given that size reduction i am going to be dissappointed with the loss of a lot of detail. This is what rendo is trying to avoid.

Pass no temptation lightly by, for one never knows when it may pass again!