Well, the OS does run on all 3 devices. But by supported, I mean, not Jolla will port it. But they confirmed that it ran on the N900, but not as functional as on the N9/50. They aren't going to be ported by Jolla (To Jolla, support means port, which they won't do). They said that a hacker out there is doing it, he will port it as soon as he reveals the tutorials. They only tried it to confirm that it works on those 3. A hacker will soon port it to our devices.
Concerning the N900, it will work but it doesn't confirm or assure that the device won't lead to further Hardware faults due to porting it. It will make the HW just weaker, it won't be that good to be daily used. But for the N9 and N950, it will be smooth enough that you can even replace Harmattan with it