Packing with ice packs is a standard component of TTM, generally in conjunction with other therapies (chilled IV fluids, etc). It's also the easiest step to take in a prehospital setting, which is why we do (this is a thing I've done with maybe a dozen patients over the past few years). I'm not just making this stuff up, it's based on actual treatment protocols and algorithms.
We're talking about a naked, sedated patient (so they're already losing a lot of heat due to simple convection, and there's no heat being generated by skeletal muscle movements). The body's metabolism has slowed significantly, and it's producing anywhere near the normal level of "baseline" heat.
