it’s a number that i pulled out of my ass but yes, it’s about annualized failure rate. If the average battery breaks after 9 years, then it means there would be a lot of free replacements under the 8 year warranty. Like for hard drives they give 3 years warranty but it’s normal to assume that it lasts 10-15 years and not break immediately after 3 years (exception: [western digital inserted a timebomb[(https://www.reddit.com/r/synology/comments/13uyk41/wd_red_pluspro_issue_drive_being_markd_as_warning/) to mark their drives as faulty via SMART as soon as the warranty is expired)
for the mitsubishi imiev most batteries are still working (different chemistry from modern ones) even if the last ones have been made in 2013
This is a bit different than MTBF like on harddrives. Batteries are usually warrantied to 80% capacity because it’s a wear thing, not a random chance of complete failure. A battery isn’t going to last twice as long as another one by chance, this is all about determining the average or worst-case operating range the battery will be in and using that to figure out a warranty period where they think all cars will fall within.
