I love this quote and it is the first one written in my B.O.W. (Book of Wisdom). I take this as implying that genius is the top, the finish line, can't get any smarter. Thus once you have reached the level of genius, you are doomed to stop growing, and once you stop growing there's nowhere to go but down. Therefore once we start to view ourselves at that "genius" level in whatever field/subject we choose, we are then cutting ourselves off from even entertaining the need to learn more... thus making that our stopping point, and we are now stagnant, allowing others to surpass us as time goes by and the world around us changes. If we are stagnant, we cannot evolve with those changes, thus limiting our options. Therefore... genius has limits.
I also take this as a warning and reminder, that stupidity is limitless.
Sensei-Guro T. Kent Nelson