Why does property access work when subsref is overriden?

Below is a class which subclasses double and does no more than storing the extra instance variable y, initialized to 123.

classdef foo < double
    properties
        y;
    end
    
    methods
        function obj = foo(x)
            obj = obj@double(x);
            obj.y = 123;
        end
        
        function x_ = subsref(obj, S)
             switch S.type
                case "()"
                    x = double(obj);
                    x_ = foo(x(S.subs{:}));
                case "."
                    % no isprop check - propogate any errors upwards
                    % what exactly does the property access call?
                    x_ = obj.(S.subs);
                    return;
            end
        end
    end
end

Example of usage:

a = foo([4 5 6]);
double(a) % displays 4 5 6
a.y % displays 123

I am confused on why x_ = obj.(S.subs) is permitted and works when S.subs == "y". I believe that obj.(S.subs) implicitly calls subsref(obj, substruct('.', S.subs), but such calls to subsref aren't allowed for builtin subclasses, e.g. double subclasses. What is actually going on here?