Shimpaku is a made-up compound word or phrase in Japanese. Apparently, Shin is “authentic” and paku is “oak”, meaning the “genuine article”. This is how the junipers were described by early collectors when they were discovered growing on the cliffs...the real deal. So it’s a made-up term, and Clyde is right on this, botanically, they’re all Juniperus
chinensis. Although I have no idea if any of the Kishu, Itoigawa, Kiyozuru, Tohoko etc. regional variants have been identified as separate cultivars.
I found an article about the discovery of Shimpakus 15 years ago in the WBFF website and actually printed it. A few years ago I went looking for it and the authors, but the site was defunct and the article was gone, I was able to recreate the article using the Wayback Machine for photos and my printed copy for text. It is now on my blog site, and is a great read, and explains the term Shimpaku, as well as Itoigawa and Kishu. Check it out:
https://nebaribonsai.wordpress.com/the-shimpaku-juniper-its-secret-history-from-kindai-shuppan-2003/
BTW, to the OP, my bet would be on “not Kishu” as well.