In the book Kelvin accepts that he's standing in front of a godlike creature that the human mind will never understand, he surrender, lands on the planet's surface hoping that "time of cruel miracles was not past".

The acceptance is not of the illusion itself, but of the fact that we would never understand the whys, the rational thinking and scientific positivism have become the real illusion, all is lost, Solaris won, there is only the hope that Solaris will keep making "cruel miracles".

He doesn't understand them anymore than he understood the people who were created, or the real people.

I always thought that was the point, to ask if his mind couldn't be used to create the other person....then did he really know them? Can anyone.