I disagree wholeheartedly (except for your last sentence). Books such as Eden and Fiasco are firmly rooted in the tradition of the sci-fi adventure, even if he adds additional philosophical and scientific concerns that were completely his own. But Lem's work is mostly explicitly grounded in scifi.

As an aside, I find Lem's prose to be rather dry, most of the time (though The Cyberiad is a notable exception). I don't know if this is something one can blame on the translators. In his scientist-focused novels, especially Solaris and The Futurological Congress, this works well; also, those are short works where the stodgy prose doesn't overwhelm. In other, longer works such as Fiasco and His Master's Voice, I find the prose almost unbearable. (Those latter two could really use some editing, too. Fiasco opens up with a seemingly endless chapter wherein a guy wanders around on an ice planet in an exosuit, and the prose catalogues nearly every single step the guy takes; it's infuritating and sleep-inducing at the same time.)