Interesting: The main description says “This is a flag used with llGetObjectDetails to get the Avatar_Rendering_Cost of an avatar, based on values reported by nearby viewers. If no data is available, -1 is returned. The maximum render weight stored by the simulator is 500000. When called against an object, 0 is returned.”
The implication is, the server only knows if a viewer has recently reported the rendering cost to the server. That (top of page) description seems more specific than the one you copied from the chart. Helpful?