What does "a perfect mirror" even mean? The mirrors produced for the NRO were very good but they're not alien artifacts indicating some paradigm shifting techn

What does "a perfect mirror" even mean? The mirrors produced for the NRO were very good but they're not alien artifacts indicating some paradigm shifting technology gap between government and the private sector. They are roughly comparable to the mirrors produced for astronomical purposes in the same era. Better because they had better funding, but not so much better that anybody should deduce that aliens had anything to do with it.