Maybe he should have his speakers measured!
Slight problem for a regular multimeter to even just to measure the "DC" resistance though, assuming the schematic is accurate. The LF section will measure very high value, in mega ohms anyway, due to that gigantic series capacitor. It can be done, but need a little more work. I am really quite sure this is one case where probably some DIY individual took some measurements, posted that curve and then it kept getting circulate around, with or without editing and because those big woofers and mid woofers do draw a lot of current, killed a bunch of amps, and awarded the killer title, and then in turns make the impossible 0.8 ohm dip look credible.
To me, if Infinity specified 4-6 ohms nominal, that just backs up what the schematic shows is believable. It would have been totally and utterly insane for the Infinity designers/engineers the Kappa series to have dips below one ohm at more than one place, and with a curve like the one circulated, yet still claim 4-6 ohms nominal. In fact, that would be more than insane, more like false advertisement to the extreme.