Yes, I read the article. It specifically says the sediment was dated. I'm just curious how they're certain the placement of the crystals dates to the same time.
This dating method basically dates the last time the sediments were directly exposed to ionizing radiation (i.e. sunlight). The key here is that it sets a minimum age. Things can be older and have their "clocks reset", but the decay constant is fixed, so we can always determine the last time the object or sediment was exposed. There are a couple minor issues with it, but it's better (and sometimes cheaper!) than radioisotope dating. You can be pretty confident in OSL/TL dates more than a couple thousand years old.
