The open port seems like a likely explanation. The thermal effect of the DLC finish is an interesting idea from a physics perspective, but I doubt there is a significant difference. See Kirchhoff’s law that predicts thermal energy absorption and emission due to radiation.
https://en.m.wikipedia.org/wiki/Kirc...rmal_radiation
At only 150 deg F, emission is largely in the IR, and most objects are nearly black bodies at that wavelength. However, polished teapots retain heat better than black rough ones because a mirror finish’s emissivity is lower, so there could be something to your idea.