The PW2 *is* a 6th gen Kindle. Generations are within all Kindles since the original. Sequence/iteration numbers are specific to a model of Kindle. The Kindle Paperwhite 2 is the second of the Paperwhite, but was *also* the only model in the 6th generation of Kindles.
Generation roughly aligns to years since the original was released, with a few times when a year went without any new models (2008, 2020, 2023).
https://en.wikipedia.org/wiki/Templa..._Kindle_models