The question he is answering is:

    "After memory safety, what do you think is the next big step for compiled languages to take?"

It might be that he thinks it has successfully pushed the state of the art in terms of regions and affine type systems, but that there are still things outside that to explore and push. 'Parametric mutability' seems to be at least in part a response to some patterns that have emerged in Rust due to the lack of this feature.