In the iconic photo of the device under test, one can see the apparatus with the red-hot glowing wires visible through the translucent 3mm thick alumina casing.
This is a significant observation, because it's the principle source of evidence that the thin alumina shell is translucent and not 100% opaque.
Why does that matter? It matters because the IR camera equipment that is used to reckon the heat coming out of the device assumes that the alumina shell is an isothermal black body radiator operating at the emissivity of alumina at that temperature. But that conveniently simple energy budget model breaks down if the alumina casing is not 100% opaque. As can be seen in the photograph, some of the photons from the interior apparatus are being transmitted through the translucent shell, rather than being absorbed by it. When those directly transmitted photons impinge upon the IR camera, which is calibrated for the emissivity of alumina, the calculation model incorrectly assumes the alumina shell itself is glowing red hot in accordance with a black body radiation model. This results in a sizable systematic error in reckoning the heat being produced by the device.
Imagine looking at an ordinary household light fixture with a typical translucent shade around the bulb. The filament inside the bulb is at an incandescent temperature, but it also has a very small surface area. When you look at the light fixture with the translucent shade in place, you see those same photons, but now they appear to come from the large surface of the translucent shade. If you imagine the shade to be the originating source of those photons, in accordance with a black body radiation model, you (incorrectly) deduce that the shade itself is glowing at that same incandescent temperature. Since the shade has orders of magnitude more surface area than the filament inside the light bulb, you end up concluding (incorrectly) that an enormous amount of heat is being produced.
In short, the experimenters have to reckon the translucency of the 3mm alumina shell that encases the apparatus, and adopt a corresponding energy budget model. Since that's not practical, they need to encase the entire apparatus in a fully opaque isothermal shell, so as to be able to properly apply their isothermal black body radiation measurement technique to the system.