That's a constraint on thermal power, not on temperature (?). Hotter objects radiate faster (T^4 scaling even), so it's *easier* to cool them in a vacuum, all else being equal.
MMRTG [0] dissipates 2,000 W, in a package that only has several m^2 of radiator fins. That's >100x more efficient than the ISS (1 W/m^2). ISS' radiators have to operate below room temperature; MMRTG fins [1] are in the 100 °C - 200 °C range. Higher temperature -> higher heat dissipation per fin area.
(By the way, the comment we're replying to asks about nuclear quadcopters on Titan [2], which is in dense atmosphere not a vacuum!)
MMRTG [0] dissipates 2,000 W, in a package that only has several m^2 of radiator fins. That's >100x more efficient than the ISS (1 W/m^2). ISS' radiators have to operate below room temperature; MMRTG fins [1] are in the 100 °C - 200 °C range. Higher temperature -> higher heat dissipation per fin area.
(By the way, the comment we're replying to asks about nuclear quadcopters on Titan [2], which is in dense atmosphere not a vacuum!)
[0] https://en.wikipedia.org/wiki/Multi-mission_radioisotope_the...
[1] https://sci-hub.se/10.1063/1.2169255
[2] https://en.wikipedia.org/wiki/Dragonfly_(spacecraft)