That is misleading. Debian seems to have changed earlier, but not done an official release for some time after the change; Ubuntu took, as they always do, this unreleased version of Debian and released it as the new version of Ubuntu.
No. Even if you don't agree that a stable release is the correct check point, that means the change was done during Squeeze's testing cycle and thus Lenny's lifetime, which started in 2009. Fact remains that Ubuntu switched years before Debian, as a number of sources confirm such as this LWN post from July 29, 2009[0] noting that, at that time, dash remained a topic of discussion on Debian mailing lists (and an unrealised goal for Lenny) whereas Ubuntu had switched 3 years prior.
> Ubuntu took, as they always do, this unreleased version of Debian and released it as the new version of Ubuntu.
That's not just "misleading", that's outright bullshit. Ubuntu switched to dash as /bin/sh independently from Debian and regardless of what Debian did.
I stand corrected. I did try to do some research, and what little I could find seemed to support my recollection, but apparently I was confused and failed in my research.
[0] https://wiki.debian.org/Shell
[1] https://wiki.ubuntu.com/DashAsBinSh