He goes so far as to say he finds Rebbi to be more correct than the Chachamim but not because of his stated reason, rather because of science. He basically insinuates the Rakia is nothing more than a fantasy believed in by Chazal which has since been disproven. He does try to show respect when stating this.
I’m progressing through the book and just want to point out that this critique isn’t fair at all IMO. This isn’t *him* who makes that point. Rav Hai Gaon and Rav Shrira Gaon (aka the Geonim) say that this is why they disregard the braissa the RT based his shitta on. They literally say that we can’t accept it because we know that the sun isn’t blocked by any Rakia, and they give other reasons, based on science, than Rebi’s stated reason.