The word "observable" does not depend on whether modern technology actually allows detection of radiation from an object in this region. It simply means that it is possible "in principle" for light or other signals from the object to reach an observer on Earth. In practice, there is much that we cannot see. We can see light only from as far back as when particles were first able to emit photons that were not quickly re-absorbed by other particles. Before then, the universe was filled with a plasma that was opaque to photons. 