That is not correct. The "distinction" between ch and ts, as well as s and sh is the one that has supposedly been lost through time.
ch and sh are used when it corresponds to Mandarin zh, ch and sh, and z c s are used when they correspond to Mandarin z c s or j q x where they correspond to the Cantonese z c s.
The one they wrote about above is not a distinction, nor is it the historical one the above commentors have been looking for.
ch and sh are used when it corresponds to Mandarin zh, ch and sh, and z c s are used when they correspond to Mandarin z c s or j q x where they correspond to the Cantonese z c s.
The one they wrote about above is not a distinction, nor is it the historical one the above commentors have been looking for.