The Dalai Lama once visited a remote monastery high in the Himalayas and observed monks who performed similar feats with their body temperature.
He personally witnessed and wrote in “The Art of Happiness” about monks sitting outside in a snowstorm, robeless. They were then draped with wet towels and, using a type of meditation, could warm themselves to such a degree that the towels became dry!
You can’t really get a better witness than the Dalai Lama.