Solaris is focused around a planetary ocean that's sentient in some manner too alien for humans to understand. That's how Lem sets up the story, because he likes to explore failures to communicate and what it would mean to encounter the truly alien. But it is his setup, and not the result of an actual alien encounter. We don't know whether it would be similarly impossible to communicate with aliens. Maybe Sagan and the SETI folks assume correctly that it is possible.