This is a belief that people have which is unsupported by science. Intelligence is a thoroughly tested and measured trait, and it can be measured with accuracy and reliability.
I think this misconception is primarily due to the biased and racist history to intelligence testing; error involved with any sort of testing; and general glacial pace at which knowledge moves from cognitive science to education, public policy, and classroom usage. There can also be distortion from what cognitive science and intelligence research supports and how that information is used to create policy.
This part, I do agree with. It seems pretty clear that the components necessary for life as we know it exist in lots of places. What is unknown is the likelihood of those things combining to create life. Is earth the 1 in 10XXXXX chance it happened, or is it nearly inevitable?
I think people also don’t understand the time and distance scales that need to be overcome to detect anything. Even with the most sensitive telescopes we can imagine, would an alien 1000 light years away be able to conclude earth has intelligent life? They could certainly see an oxygen atmosphere, and maybe based on the sun’s spectrum and green reflections on earth make some conclusions about photosynthesis, but that’s an incredibly long way from detecting the Sung dynasty or whoever.