Eh, it's double the radius of Jupiter, which makes it about one fifth that of the sun, right? So the apparent BB area is a 25 times less than a star, which is small but not crazy small. More importantly, the amount of light from the parent star filtering through the planet's atmosphere would seem to be much less than the amount emitted directly by the planet, since the apparent surface of the former is only a thin ring of area around the surface for the latter.

Several things: stars get much cooler than the sun, a factor of five in radius is a factor of 25 in surface area, the heat may be focused on the side pointing at the star (meaning the star masks the light), and light from the star could be overwhelming the light emitted by the planet. Astronomy is filled with orders of magnitude.

You repeated one of my own points back to me and then condescended down to me about not appreciating orders of magnitude. (I'm a physicist myself.) The whole purpose of my comment is that 25 is not that small of a factor on astronomical scales and it suggests the BB radiation of the planet itself would be detectable. As it turns out, this is the case

> As a result, [the planet] has an extremely high equilibrium temperature, assuming zero albedo and perfect heat redistribution, of ∼4050 K. This is as hot as a late K-type star, and thus we expected a large thermal emission signal, which we easily confirmed with our z'-band detection of the secondary eclipse with a depth of ∼ 0.1% (Figure 1). This measurement implies an even hotter day-side temperature of ∼ 4600 ± 150 K, likely indicating poor redistribution of energy to the night side of the planet and a temperature closer to that of a mid-K star.

https://arxiv.org/pdf/1706.06723

Note also that, contra your suggestion that the planet might just be too cold on night side to have significant BB radiation, the night-side temperature is only 15% lower than the mean.