Seems like if that was Apple's goal, it would be easier for them to just reduce the quoted battery life of the phone, limit the charge to 80% for everyone, and relabel the 0-80 scale to 0-100.
Presumably, they've already effectively done this by choosing voltage amounts that balance warranty repairs with battery life. e.g. the iPhone 15 charges to a max of 4.48V, but the battery presumably physically supports some higher voltage for "100%" that would give more capacity and less lifespan. (Notably, different iPhone models have different voltage ranges.)
"reduce the quoted battery life of the phone" - This is the bit that presumably they don't want to do.
Reducing the top-line specs of the phone means that it might fall down in comparisons to others in a spec-comparison.
I presume they would rather move the conversation into customer-land, i.e. once you've bought your phone they talk about how they are acting to preserve your expensive device.
Presumably, they've already effectively done this by choosing voltage amounts that balance warranty repairs with battery life. e.g. the iPhone 15 charges to a max of 4.48V, but the battery presumably physically supports some higher voltage for "100%" that would give more capacity and less lifespan. (Notably, different iPhone models have different voltage ranges.)