Indeed; I've replaced the laptop since then, so I don't think I have the log, but I believe it very explicitly logged it was the CPU temperature provoking it before actually shutting down.
Is this normally logged anywhere? You may have just saved me RMAing a laptop.
I had a computer with a broken GPU fan and it thermally shut off. There where kernel log entries about temperature warnings, though I don't remember if it mentioned which component. Systemd should preserve them before the system shuts off.