Given that TIR1 records are stored as 10-bit values (i.e., in the range 0 to 1023) and the color scale maxes out at 939, I suspect that they are just plotting the raw count data. This is normally transformed into radiances by applying a linear scale and offset to put the data in physical units.

I had a quick look at the equivalent 10.8 micron image from Meteosat over the Indian Ocean (see below) and I get a similar count range and a similar image when I apply their color scale. I also suspect that they've inverted their data, e.g., plotted count_inv = 1024 - count, to fit with the viewers' expectation that cold clouds appear white.