Many motherboard manufacturers will say things like this. Experience will tell you that they will do it everytime their product does not support or has problems with what is being asked.
For instance, MSI and Gigabyte told me in November 2007 that Phenom did not support DDR2-1066 either, just because their motherboard had severe problems with it. It's a defensive method to save their own butt. After a few BIOs updates their motherboards worked perfectly fine with DDR2-1066. Later I learnt they actually were ignorant of how to get DDR2-1066 working on the motherboards uptil then.
First of all, for Gigabyte say Phenom itself does not support DDR2-1000 is wrong and based on no evidence. I and many others online and offline that I know personally have ran DDR2-1000 even in 2x2GB kits since a while now. However, some motherboards and/or BIOS versions do have problems running DDR2-1000 kits, severe problems, much more than running any other specific rated speed. The DFI 790FX motherboards have severe problems with DDR2-1000 kits for instance but it also suffers incompatibility with many DDR2-1066 kits.
I don't know an official answer, have never asked for it, sorry. Official answers will be based on caution and minimizing problems rather than if any motherboard can possibly run it perfectly. Why not email AMD directly?
Although if you're asking about what the K10 IMC is programmed to run, then it's DDR2-533, DDR2-667, DDR2-800, DDR2-1066, DDR3-1066, DDR3-1333 and in some cases DDR3-1600. Nothing else is set in stone and it's more of a trial and error situation.