Huh, yeah. If you take their numbers and blindly plug it in you get 197 instead of the 84.4 that they have written. So according to their math the junction temperature is hotter with a heatsink. This is because the silicon insulator's thermal resistance is as high as the junction-to-ambient resistance without any heatsink.
THere is probably a typo somewhere. You get a closer answer if you use 6.5C/W or 0.65C/W for the silicon insulator, but still not exactly their answer. Either way, the formula is right. It's just that their math is wrong or their data for the silicon insulator is wrong, or the silicon insulator is so bad that you might as well not use a heatsink at all (or at least not use the silicon insulator).