Which video was this?
Are you sure he's not using Dynamic Topology before retopology, or are you watching a really old video (Blender 2.67)?
I cannot say I am entirely sure which high-res issue are you referring to, but the retopology is used to offer optimized detail, replacing the previously dense topology. Then you can add fine detail with the multires on top of the retopoed mesh.
I hope that answers your question. :S