This chart is measurement data taken from the very useful ink percentage test that you can print using the QTR RIP. And we have found that the colors are additive. ( I use we, because Michael Mutmansky and I sort of put this thing through the wringer about two years ago and he had this hunch that the colors might additive, and sure enough, the testing proved it to be true)
The ink load remark was to point out that if you print out 100% of each ink color, you will have a big puddle of ink on your pictorico. So you need to take into account the cumulative ink load (in percent) for all the colors you are using to avoid over-inking the substrate you are printing on.