At around 20:23 youtried to select the border and 'accidentally' managed to do so,but what should be done is to ALT Select an Edge and then ALT Select that same Edge again to select the whole border.
(I accidentally stumbled upon that trick in the Manual:
"All boundary edges can be selected by performing a second loop select action on a boundary edge.
This can be useful for selecting boundaries for meshes that include triangles and n-gons, where loop select would not otherwise select the full boundary.")
