If you want to be part of Kindle Unlimited you have to give worldwide exclusivity to Kindle Unlimited, and can't have ever published your eBook on another platform.
Even if I wanted to join, Kindle Unlimited is not offered here. I can't even buy the eBook from Amazon.
My daughter wanted a book that was kindle exclusive in the US, and I found I could purchase an epub from another store by paying in euros and claiming to live in Europe. Needless to say I did this without a VPN and without leaving San Francisco. The book was still in English.
But I wonder if the reason for that little hoop was because of Kindle Unlimited.
It's not just that. E.g. Cooper and Hutchinson's edition of Plato's complete works, available on Kindle for $31.[1] Or on another tack, Yudkowski's recent If Anyone Builds It, Everyone Dies.[2] Neither book is on Unlimited, and I couldn't find either one on Kobo. I struck out half a dozen times in a row and finally gave up.
While it's true I don't see that version of Plato's complete works (in fact, I don't see any Hackett Publishing books), the book on AI[1] is available.
Even if I wanted to join, Kindle Unlimited is not offered here. I can't even buy the eBook from Amazon.