I happen to know a thing or two about the Android compatibility technologies available for Tizen. At least one of them is a port of the Dalvik bits from AOSP, so, inherently, compatibility is really very good. On top of that, one can even blend the Tizen "Home" and task switching with the Android back-stack and task management. That's tricky because "task" in Android doesn't necessarily map to one process. It's an interesting engineering problem and I hope, for entirely selfish reasons, Samsung makes a good choice there.
However, that doesn't make the resulting product a threat to Android. It would start on one or two handsets. It would lack Google's ecosystem, and building the right alliances in the right regions is a heck of a product marketing problem. It would have to be refined over multiple product generations.
Right now, RIM's whole business rests on 1% market share. That's a tall mountain to climb starting from zero.
So, what I'm saying is, even under optimal circumstances regarding Tizen, Google is rational to think they don't have to consider Samsung a threat this year, or for a few years.