Thanks! I'm sorry I didn't include that link. They say of the dating method:

> Younger samples are dated using single-grain quartz optically stimulated luminescence (OSL) and older samples by postinfrared infrared stimulated luminescence (pIR IRSL) from potassium-rich feldspars (Methods and Supplementary Information Section 2). The pIR IRSL approach used extensively in recent years²⁵,²⁶ does not suffer the problems that can generate large uncertainties associated with thermally transferred OSL (TT-OSL), as seen at Site C North (Fig. 1b)²⁰.

I had never heard of this archæological dating method before, but Wikipedia comes through as usual: https://en.wikipedia.org/wiki/Optically_stimulated_luminesce...

Not having read the full paper, I don't understand why they think the date at which the sand around the wood cooled from magma temperatures is relevant to when the carpenter cut the logs? Or maybe they're assuming the sand was exposed to the sun and optically bleached around the time of the carpenter, so any trapped charge is from after that? https://insu.hal.science/insu-03418831/file/MurrayEtAl-2021-... looks potentially relevant.