the ONLY reason I can think they would say that is because they do not have a boot for the extended ones. not putting a boot on it ...race or street is foolish in my opinion. The grease will draw dirt/dust like a magnet.
I had the extended ball joints on my car all last years. I made some 1" long X 3/4"id spacers that slid over the lower part to hold the boot up. just a little hot rod ingenuity.....some would call it ******* engineering