A quick look at the book makes me hesitant to accept the diagram as a claim that the line is joseki as opposed to an illustration of White's ability to crosscut against the hane. Note that the last two thirds of the book consist of about forty examples of handicap fuseki. And how many times does White attach underneath in those forty examples? None as far as I could see (feel free to correct me if I missed it).
It is certainly true (and highly regrettable) that most games have not
yet (

) made it into the databases. Despite that my current database spits out 733 examples of the large-knight response to the small knight approach in an otherwise empty 10x10 quadrant played between 1660 (to exclude most but not all old Chinese games) and 1884 the date of the book. How many times does White attach underneath as the next play to occur in that quadrant (not necessary the very next play)? Again, there are no games where this occurs. Remember that the early games in the databases are highly biased in concentrating on the famous historical players. So if we want to think that the relevant games are just missing from the databases, we have to also think that there was a 'joseki' in use that was not used by people like Dosaku, Jowa, Shusaku, Shuho, and so on. This is questionable because the list of 13 games where the attachment
did appear, as a response to a pincer or at least with other stones close by, looks like this:
Honinbo Dosaku - Yasui Chitetsu (B), 1673-81-00
Yasui Senkaku - Komatsu Kaizen (B), 1751-00-00
Kojima Doshi - Yotsutsujimae Dainagon (B)
Honinbo Jowa - Ito Matsujiro (W), 1822-04-08
Ota Yuzo - Ono Kaibee (W), 1839-04-28
Ota Yuzo - Yasuda Shusaku (B), 1842-08-13
Ota Yuzo - Yasuda Shusaku (B), 1842-08-17
Ota Yuzo - Yasuda Shusaku (W), 1842-08-20
Honinbo Shuwa - Mitamura Kisaburo (B), 1845-04-17
Ota Yuzo - Murase Yakichi (-), 1854-06-07
Murase Yakichi - Kajino Tsunejiro (W), 1858-11-13
Murase Shuho - Kobayashi Tetsujiro (B), 1866-00-00
Murase Shuho - Yamazaki Sotosaburo (B), 1884-05-00