The wiring would of course carry 30mA, however if something at 230V contacts an earthed wire, the fault current will be many thousands of times greater than that.
And therefore trip the RCB removing all 230 power.
If the wire of an SELV circuit comes into contact with 230 volt then the entire SELV circuit is at 230 volts above earth potential and there is no way that this can be detected and the 230 volts removed automatically. In the case of a door bell button that is wet with rain that could result in electric shock to some one pressing the button.
SELV is specified as the circuit being "Safe" or "Separated" in the event of any single fault. And for most faults that is true. But is a melt out of insulation on a 230 volt conductor that also melts the insulation on an SELV conductor and results in the SELV conductor becoming live from the damaged 230 volt conductor a single fault ? Apparently it can be considered as two faults. Two separate wires have lost insulation at the same time so two faults and outside the single fault specification of the SELV.