When you access "dl", you will find it to be a matrix. In your case "dl" contains 16 elements, each of which corresponds to the edge number you see in the original figure. I used this idea to extract the required edges from the given figure and plot the figure you are looking for. Please find the code attached below:
rect1 = [3
4
0.1789
0.40735
0.5205
0.2286
0.14231
0.32402
0
0];
C1=[1
0
0
0.2286];
C2=[1
0
0
0.5205];
C1 = [C1;zeros(length(rect1) - length(C1),1)];
C2 = [C2;zeros(length(rect1) - length(C2),1)];
gd = [rect1,C1,C2];
ns = char('rect1','C1','C2');
ns = ns';
sf='(rect1+C2)-C1';
[dl,bt] = decsg(gd,sf,ns);
pdegplot(dl(:,[1 2 8 13]),"EdgeLabels","on","FaceLabels","on")
% Do note the Warning Message, and ensure that Meshing is possible for any
% future use
xlim([0.1,0.6])
axis equal
Note: Ideally, it is advised to find the boundary conventionally and then use it to plot and mesh.
