I actually got an answer from Kara saying "All Goodreads reviews will display on Goodreads on Kindle, as it is part of Goodreads. This is similar to how all reviews show on the Goodreads iOS app and the Goodreads Android app. It's worth noting that you have to be a member of Goodreads to access Goodreads on Kindle, which includes the Goodreads book pages.
As Elizabeth mentioned, the third-party sharing box will be grayed out by default if your profile is private. If you set your profile to private, we automatically set your reviews to only appear on Goodreads." (Emphasis mine)
I have also asked in additional to the original post Kara answered if private profiles being set to "only appear on Goodreads" meant "only appear on Goodreads and the kindle now that kindle is part of Goodreads" -- I was a bit surprised to get a quick answer or I might have phrased that better. I plan on updating post when answered (or if you know how flaky I can be, click the source link to check for yourself).
Screenshot of original post/question: