A common joke used by both dyslexics and those who mock them is “lysdexic untie”, which is how someone suffering from that condition may spell “dyslexics unite.”
The joke that the Author is making is that a group of dyslexics, wishing to make a self-deprecating tshirt, may actually reverse the reversed phrase. This would produce the correct spelling in the end, thus defeating the purpose of the tshirt.