I will freely and fully admit that I have no ability to calculate carbon fluxes from stable isotope carbon ratios. I can only provide numbers from people that purport to know how to do it with "box models" and the like. My analogy was primarily intended to show that in a world with nearly equal natural input and output fluxes of CO2 to the atmosphere, the addition of fossil fuel CO2 is entirely responsible for the increase -- as seen. And I've noted (a few times now to drrocket, and elsewhere) that if there were no human activities, the net CO2 flux would be out of the atmosphere. THEN (on a very long, long time-scale) we might have to worry about another cool period.
The nearly equal is an assumption, and their magnitudes are not well understood either. With those assumptions, a net increase from anthro sources would be valid. But obviously the mechanisms and precision for CO2 as a whole is difficult to pin down, so that's why they use the isotopes. The trouble is, I have yet to see any isotope analysis that yields any numbers for the human component. Your favorite site for example: http://www.realclimate.org/index.php?p=87 goes on and on about how the decrease in ratio is "exactly what we expect if the increased CO2 is in fact due to fossil fuel burning" without any numerical analysis to back up that statement. Zip.