It says they are using old-fashioned, careful, formal pronunciation. Fifty years ago in the United States it would have been a marker of "educated" speech.
At one time, centuries ago, the "wh-" words were actually spelled with the h first, "hwen" instead of "when." I don't know why the order got reversed.
The American Heritage dictionary lists both pronunciations: "wĕn," and "hwĕn," so the compilers believe both are still current. I will have to start listening carefully. Notice that it isn't necessarily an all-or-nothing thing; the "h" can be aspirated lightly.