I would believe that the seed library was keeping the seeds in good storage conditions. I would just sow them normally which is around 65F soil temperature though lower is fine. Just barely cover the seeds and keep moist.

I think the half-life of lettuce seed is something like 5 years under dry, cool storage conditions. The secret to germinating old seed is to simply accept the fact that a lot of it won't.

I direct seeded some lettuce seeds from 2016 this spring and they did not germinate - at all.

Had to re-plant and the newer (though not "new") seeds did germinate, from 2018.

It might depend on how the seed was stored. I routinely germinate very old seed, from herbs like basil and parsley, to beans and peas, to assorted flowers. This Spring I sowed the remainder of a packet of snow peas that I purchased from Johnnys in 1992, no kidding, it was dated. Have gotten about 40-50% germination too.

I've stored almost all my seed in the refrigerator since the 1980s. One entire produce drawer is always full of seed, and sometimes part of the bottom shelf too. The cold temperature supposedly slows the metabolism of the seed embryo and extends the lifespan of the seed by up to 10X longer.

The advice about seed lifetime is all over the map, though obviously cold temperatures help. I just found a curious post in Burpee that says that "Spinach, lettuce, parsnip, and corn seeds are generally only viable for about a year." Sounds kinda strange to me. The only seeds I know of that are toast after a single year are onions, and they don't mention those.

https://www.burpee.com/gardenadvicecenter/areas-of-interest/flower-gardening/saving-seed-packets/article10846.html

Put the lettuce seeds in a jar and stick them in the refrigerator for a few days before germinating. The brief stratification can improve germination.