I don’t want to discredit this, since I don’t know your source, but linguistically that seems quite unlikely for a consonant introduced three thousand years ago to have influence on the modern Cornish place names. Three thousand years ago, the Celtic languages hadn’t even split from proto-Celtic, so any influence on Cornish should have also had influenced on the rest of the Celtic languages. In addition, the sound /z/ is a very common one across languages (so it does indicated Phoenician influence), and they didn’t have a script then to write the place names (so the orthographic letter z doesn’t mean anything).