4) due to greater cell voltage differences across cells in the 2017-2019 batteries, those batteries more often had lower capacity than nominal when new, even before any capacity loss due to use or age.I think there are a lot of 2020+ drivers like @CHASBOLT. There is a lot more incentive for 2017-2019 owners because of 1) greater risk (although unclear how much), 2) existing batteries for those years have likely degraded more under normal use, and 3) greater capacity than when new.
Also, the risk of fire difference for different lots of batteries was also considerable. From publicly known fires, it looks like about 1 in 300 or so of the 2019 cars with Korea-made batteries with build dates in October 2018 have burned already. Hence there are certain very high risk lots of batteries that GM presumably wanted to get replaced as soon as possible. On the other hand, the defects and risk of fire for 2020-2022 appears much lower.