Nonsense. I have a computer that doesn't have the graphics chip required to run Windows Vista Aero and it's fancy transparent windows crap. But I can still install and run the rest of Vista.
With computers the issue is generally backward compatibility, not forward compatibility. Assuming you have the basic capacity (sufficient disk space to hold the OS, sufficient RAM to load the OS, and sufficient CPU to run the OS) the hardware, regardless of how old it is, puts constraints on the OS but it doesn't prevent the OS from running. OTOH, trying to run an old OS on new equipment often means that the equipment isn't recognized because the drivers for components do not exist in the new OS. Unlike computers, with phones you program to your hardware. Maemo 5 and 6 reflect the smartphone nature of the N900 and its successor.
It's unfortunate that I would have a better chance of running the latest Ubuntu on a 20 year old 386 than running Maemo 6 on a two year old N810.