The study you quoted only seems to address if signing helped the child learn spoken word labels about certain toys.

The (possible) benefit of signing is that the child can communicate with you about whether they are hungry, thirsty, cold, hot, have a wet diaper, etc.--not about whether the child can name different toys. The study doesn't address whether or not sign language reduces frusteration in children or whether children can learn signs for how they feel faster or slower than they can learn the same spoken words.