rejected_connections is not your current connection count vs limit, it's the number of rejected connections since Redis server launch time. During the lifetime of your redis server, rejected_connections is incremented each time a new client try to connect to redis but reach the max client limit.
So indeed, currently the redis server may have 7 current connected clients and a max client count to 30 but it's rejected_connections may be at 7. Note that if you restart your redis server, rejected_connections will go back to 0.
That's why you receive the Redsmin alert, that's why you may want to edit it to 7 instead of 0, so you will be only alerted next time your redis server reject another connection.