The thing is "dyslexia" is a label rather like "blindness" - it indicates that there is an inability to do a particular thing, but says nothing whatsoever about which of a large number of underlying causes is involved.

And the trouble - the "myth" in the words of the programme, and the book that lies behind it, is that often people take that word and treat it as if it was a diagnosis that in itself identified what was at the root of the reading difficulty. (Or at the other extreme take it that there is something mythical about the fact that people have the difficulties that have resulted in the label being attached to them. Which is self evident nonsense.