For discussions of longevity actual capacity is relevant because where the full/empty lines are drawn has a big effect on how much wear and tear gets put on the pack in a cycle. A literal 100% charge is very different than a nominal 100% charge.
The degree to which each percentage point effects longevity surely varies depending on battery design. It would be a lot more useful it describe it in terms of recharge-discharge cycles than to say "it is limited to charge only up to 90%" and let the listener try to infer what that means about lifetime.
