Author here — maybe it was dumb of me to try, but it didn’t come close to a bad temperature. I get the feeling that the BIOS doesn’t stress the CPU very much. Of course, I would never try to boot an OS or let it sit for much longer than I did.
The fact that it had a heatspreader probably made the heating much less rapid than it could've been with a bare-die CPU like an old Athlon or some of the mobile ones.
It's hard to imagine it using SMP, so only one core should be active, and it's not likely to use many of the advanced functional units. I imagine you have a pretty high "thermal margin" in that run state.