This was documented on the same page but I have missed it.
The trick is mentioned here:
Get Property Value from Object Array
If the input to the MEX function is an object array, call getProperty with index of the object in the array whose property value you want to get. For example, this code snippet returns the value of the Name property for the fourth element in the object array, objectArray.
matlab::data::Array objectArray(inputs[0]);
matlab::data::CharArray propName = matlabPtr->getProperty(objectArray, 3, u"Name");