You wouldn’t lose the details and general shape of the original high-res mesh.
At 5:04 they show use of a shrinkwrap modifier with proper hand retopology.
Their remesh by auto means is then taken into miltires for the fine details.
What I mean is a Shrinkwrap below the Multires on the modifier stack after the retopology. In the second part of the video Grant has to re-sculpt the shape and details of the original high-res mesh, but with a Shrinkwrap he could project the retopo on the high-res mesh. If you apply the Shrinkwrap, the highest subdivision of the Multires modifier of the retopo mesh looks like the original high res-mesh.
Oh. I can only guess that they did not consider the first shaping to be that serious and worth preserving. More of a roughing out phase.
I am playing with a dragon myself at the moment and I skipped the rough sculpting then poor auto retoppo altogether. Started with the hand retoppo if you like. Made the dragon mesh by modelling.