It's like comparing oranges
It's like comparing oranges to apples. The battery formulations changed over time, as well as charging profiles, changing their heat tolerance and degradation. Early 2011/12 models had significantly more degradation than 2013-2015 (and early 2016) models.
2016 introduced the 30kW packs, which had a different set of issues, which is speculatedly related to the charge profile not slowing down the charge even when the battery is very hot during DCFC. Higher density in the packs also meant it didn't cool down as fast either.
2018 introduced the 40kW packs and now has better temperature control when the pack is very hot, leading to slower DCFC charging (and subsequently should lead to better durability of the battery, and reduced degradation).
So you can't compare the 2018 to previous models. It's very different in each iteration of the LEAF. A degradation model in one iteration does not work with another.