"!tables" is a feature of Sqlline, not Phoenix.
I don't believe Phoenix has a standard SQL syntax for listing tables (such as `show tables`). You would have to use the DatabaseMetaData API that JDBC provides. I am not sure if Zeppelin exposes that for you.
https://docs.oracle.com/javase/7/docs/api/java/sql/DatabaseMetaData.html#getTables(java.lang.String,%20java.lang.String,%20java.lang.String,%20java.lang.String[])