The machine was destroyed by the Searcher before it could even enter the outermost layer of Earth's atmosphere. Now, all yC craft would leave Earth. However, they'd know if any 'visitors' decided to poke their heads out. Moreover, the HRC and Sides of Earth would be more than enough to defend Earth. One thing was clear; the Narrikians weren't welcome on or around Earth. With their device destroyed, they'd know nothing of Earth's state of affairs. Earth's business pertained to the yC and HRC, not the Narrikians.
Out-of-Character: Which means, that you wouldn't know of Earth's radiation levels. We all know that you have selective reading syndrome. I'm just making it concrete.