There was a bit of information left out of this lesson. The Refine Method is actually quite important, particularly if you want to use relative detail and brush detail. You don’t need to avoid them in order to prevent accidentally losing detail.
The Refine Method determines whether Dyntopo adds detail (Subdivide Edges), removes detail (Collapse Edges), or does both (Subdivide Collapse). If you set your refine method to Subdivide Edges, you can use relative detail without fear of accidentally removing all of your detail with a single brush stroke.
Please do not ignore the Refine Method when using Dyntopo. Have a good day!