Chlorine Dioxide does indeed kill cysts, but the issue is time. ClO2 has shown to NOT be effective against crypto with exposure of 1/2 hour. (I think the recommended time is 4 hours to kill crytpo with ClO2).

A company that uses ClO2 as the active ingredient is trying to market its product and since time is very important to consumers they reduce the wait time and exclude crypto from the list. Of course if one waits long enough the ClO2 will break through the cysts shell, but time is not something some people are willing to sacrifice.