The real answer to this is basically that the two companies aren't all that integrated, even today. At the same time, renderman was still using ray tracing as part of it's hybrid REYES architecture.

Even so, they could have made it with renderman. Presentations saying 'this wouldn't have been possible' are more marketing than reality. It was more likely to be a combination of the right time to pitch creating a new ray tracer to see how much could be gained through batching while also having a renderer they could control and make sure was modern.