Well, my answer than is that Nathan Johnsons view is not the only one and certainly not the only thruth. But I base that on your comments as I have never seen the book.
There is some controversy in to what Kanryu Higashiaonna learned and thaught. Some say he only thaught 4 kata, sanchin sesan sanseru and pechurin. However Motobu Choki states in his book Watashi no toudi-jutsu (1932) that sanchin seisan and pechurin are kata handed down from the old Ryukyu kingdom (=before 1879, before Kanryu Higashiaonna came back from China and certainly before Kanei Uechi came back from China). The reference of origins of Goju-ryu from 1828 comes from Miyagi's Gaisetsu (1934).
Now it could be that the origin of these kata in China is from the same source but then you have to dig back very deep. Up till now nobody came up with the mother version of sanchin or sanseru or sesan. In fact P Mccarthy states in his bubishi translation that these kata's are found in crane boxing as well as tiger boxing as monk fist boxing as well as dog boxing etc... all in different forms/techniques/patterns.
There is some very interesting information around the origin of the Goju kata's in the Meibukan magazines ( www.meibukanmagazine.org
The information about importing the sai from Chinese military sources comes from a book of Kenyu Chinen 'Kobudo d'Okinawa' a student of Shinpo Matayoshi. Up till now I have not seen a Chinese civil fighting system that uses the sai as a weapon. Sai in the form as used in Okinawa, not to be confused with nunti (also called manji sai). If you can name such a system I am happy to have that information so I can research on it.
Throwing a sai to any effect is not that difficult with a bit of training. I throw them through 2 tatami into the third when throwing downward. I never said that they are a battlefield weapon, just that they were imported by Chinese military. As you know, Okinawa did not engage in war since the 14th century up till WWII. So the military would not teach battlefielmd applications, rather civil restraint applications. Okinawa did not have an army, it was under protection of the Chinese.
So I agree that they are a restraining weapon but one of the main applications in Okinawan kobudo is to throw them into your adversary. Evidence to that statement is for instance in the book 'The Weaponless Warriors' by Richard Kim in the story about Itosu Yasutsune who lived in the 19th century.
The link with the Japanese jitte is far stretched. No Okinawan kobudo authority has ever linked the sai to jitte to my knowledge.