There is no reason to suggest it's not based on WP7.
Yeah there is.
Just as I suspected, the Kin project was approved and has been in development long before Windows Phone 7. They share the same foundation, Windows CE. Although, I’m not sure which version of Windows CE Kin is based on vs. 7. But, at the moment, it’s not Windows Phone 7.
Looks like they plan on remedying that over time.
Engadget has the interview:
"We chatted with Microsoft GM Matt Bencke today to get a feel for why there isn't more technical synergy between two products that obviously share the overwhelming majority of their DNA, and basically, the Reader's Digest version of the conversation is that they intend to get there eventually.
"We're going to share more and more in terms of code," he said -- though we weren't able to nail him down to a timeline for making that happen -- ultimately agreeing that it was fair to say you'd start to see the platforms converge over time.
If we had to guess, Kin's suffering from the fact that it was approved and set in stone before Windows Phone 7 even existed in its current incarnation, and it's going to take Redmond a little while to get the projects synced. See the critical portion of the talk on video after the break.”
http://www.engadget.com/2010/04/12/microsoft-kin-and-windows-phone-7-will-share-more-technology-ov/
Additionally, this article has some interesting insight into “Pink” including the revelation it was originally based on Windows
Mobile 7 (vs. Windows CE/Windows Phone 7).
http://www.crn.com/software/220600334;jsessionid=5RHWSW32C5NT3QE1GHOSKH4ATMY32JVN?pgno=1