1: I haven't said that they have switched, i said they were in the process of switching.
2: First some background information: Renderman is the renderer Pixar uses, it was built by them and has been used since toy story 1 but has had many revisions, it has won a lot of prices and it is considered to be one of the finest renderers there is and to be pixars greatest accomplishment.
Now for the point: While the renderer (RenderMan® Pro Server™ 11.5) itself can be run on a mac, the The RenderMan® Artist Tools™ 6.0 made for use with the renderer can't be run on a mac.
3: Just because you are the founder of one thing doesn't mean you cant use someone elses work even your worst enemy. Just look at microsoft and linux...
4: While im sure that the mac will continue to be used for certain aspects in studios like pixar the rest is switching sides. It's a best out of two worlds solution.
In essence, yes i believe so...