The flirc case is a giant heat capacitor, not a heat sink. Notice that it has the only temperature curve where the temperature does not go back to baseline after the stress test. It needs a lot longer to get to equilibrium, and the final temperature is almost certainly hotter than what's shown on the graph.
I own one. To some extent you're right. My pi runs mpd and pi-hole at all times and under that relatively low load, the case is constantly warm to the touch. But it has enough surface area that I've never been worried about overheating. Refer to TFA -- a tiny little heat sink on the cpu was all that was need to keep from throttling.
I don't have the hardware to give a proper surface temp reading. I can measure the pi's reported temperature / test for throttling under extended load later, but for now: I stuck an "instant"-read thermometer intended for cooking into one of the usb ports, which was the hottest place I could find on the case. It measures 36.1°, compared to an ambient temp of 23.4°.