It may have to do with the ranking system used by the AGA in 2004, though I doubt it since this was already the 21st century.
In the previous century, there was a clearer line between kyu and dan.
For instance, in the '70s and '80s the British Go Association allowed its individual clubs to promote, without any control exerted by the national body, their members to any kyu rank the club management thought appropriate.
However, to advance from first kyu to shodan required the permission of the central committee, which operated by secret criteria relating to which dans and 1–3k players they thought were particularly strong or weak. It was a whole different ballgame to cross that divide.
Repeatedly in 20th century materials, there are references to 1k players who are thought to be too strong and overdue for promotion.
Consider the song The Strong Dutch First Kyu, originally printed in the BGA Songbook and reproduced in the AGA one.
What shall we do with the strong Dutch first kyu?
What shall we do with the strong Dutch first kyu?
What shall we do with the strong Dutch first kyu?
If they won’t promote him?Way-ay, make him shodan,
Way-ay, make him shodan,
Way-ay, make him shodan,
Why won’t they promote him?
Certainly Kerwin would have been very familiar with the traditionally meaningful kyu–dan split, since he was already in his late 50s when this advice was published (and presumably, but not definitely, written).