Ok, maybe it’s possible that they aren’t using a very focused electron beam, but usually when scientists think about using an electron beam they mean something inside of a machine like an SEM or e-beam lithograph. These only operate on small areas.
If an unfocused beam (and therefore lower energy density) can be used, then this could likely be scaled more easily. Even if a focused beam is needed, scaling may still be possible, but will likely require additional developments to create that process.