Yes you are correct that if one screw terminal fails, a simple earth test at the socket will not reveal this.

The only way to test is to physically unscrew a socket, disconnect one of the earth wires and do a earth loop resistance test. Put it all back together if the test results is satisfactory.

Having said that, I still prefer sockets with two earth terminals as even if one terminal fails in socket, you still have an earth connection to all sockets until an island of unearthed sockets occurs with a second earth terminal failure.

In this case the islanded sockets lack of earth is easier to test for and hopefully these are also still protected by an rcbo with 30mA leakage threshold.....