Indeed, the article errs on that general statement. But it seems per Wikipedia that pure-mercury thermometers become unusable at -39C, when mercury solidifies.

Indeed, the article errs on that general statement. But it seems per Wikipedia that pure-mercury thermometers become unusable at -39C, when mercury solidifies. A mercury-thallium alloy can go to -61C, but apparently alcohol thermometers are preferred for measuring extreme cold, as they work to -70C. So the actual observed thermometers on that record day – those thermometers – likely did not use mercury.