With advanced telescopes and other devices, we are discovering new planets all the time.
Some of those planets may have advanced life, perhaps even humanoid life, on them. So would that negate our discovery of those planets?
Of course not. What makes something a discovery is that prior to that event, it was not known by the prevailing culture calling it a discovery. Prior to the voyage of Columbus, the continents of Europe Asia and Africa where most of civilization resided were completely unaware of the existence of North and South America. The indigenous people living in North and South America were not part of the prevailing culture that was calling it a discovery, so therefore they were not relevant to whether or not those calling it a discovery ought to do so, just as alien life on another planet would not be relevant to astronomers calling it a discovery when that planet first got detected by them.
The indigenous peoples discovered they were not alone in the universe...
And let us not forget that Columbus “Discovered” the world was not flat — even though it already was...