There is no concept like "working for Leo". Leo is a single, recommended combination of hardware by AMD. It's Thuban Processors with the 890GX chipset and 5800 series graphics processors. Nothing not to understand. No reason to make exceptions either.
The chipsets are part of "Leo" yes. But I have not seen anything that tied the X6 to "Leo". The X6 is mentioned side by side with "Leo" and the chipsets. That does not mean "Leo" is predicated on the presence of a Thuban, X6.
If not, it may be that any current AM3 chip would qualify as "leo" providing it used the new chipsets. If that is the case, AMD "Leo" does would work with current X3 and X4 cpu's.