You are absolutely right. People should stop upvoting my post because I'm totally wrong. Not only was Cars the first movie that was done using ray tracing, they thought it was glitchy and MU which was released 6 years later was actually the first movie to be fully raytraced.

That said, the technique they did use was scanline rendering, which is still rather different from z-buffering, which is the way GPU's generally render.

Of course now I'll change my argument that Pixar is after a non-realistic cartoony feel, so that gives them more freedom.

If you look at live action movies, the CG parts in there are either painted or ray traced, they need absolute control to be able to blend them.