This study does not show a CAUSAL effect of sign language on spoken language skills. Only a correlation.
Which means that maybe the kids who were not doing well with a CI, their parents stuck with using ASL for communication. The kids who did well with their CI, parents had no need to continue ASL.
Also, in science, there are many studies published showing the effect of ASL. Some will show positive results and some will show negative results. So this is why one study does not probe anything. One must look at the aggregate of studies and look at the methodology of each in order to come to a conclusion.
If 50 studies show a positive effect of ASL on English language acquisition and one study does not show a positive effect, we should not discount all the other 50 studies in favor of the one study.