After realizing instances, the seams are kept, but their UV mapping is lost. Other users stated that you can use them with the Attribute node node in the Shader Editor. But if I want to export my model as FBX and use it in other programs, I have to unwrap it again, which ruins my stacked UV.
In 3.0 Alpha this was not a problem, but now it is not possible anymore.
It's possible by applying the modifier.
You use the attribute node to create the UVs like you've linked to in your question. Then, apply the geometry nodes modifier. On the object, select the "Object Data Properties" tab (the green once), and select you UV attributes in the Attributes box. There click the down arrow next in the attributes box (below the "+" & "-" buttons) and select convert attribute. Select mode "UV Map" and ok.
I hope this can be done in the future without applying the modifier.
