The reason everyone says that is because Apple has several important dates in the life of a product.
The date they announced a product.
The date they introduced or shipped a product.
The date they discontinued the product.
The date they actually stopped taking orders and removed it from the price list.
The date they shut down the assembly line and stopped manufacturing it.
The reason we all agree on announced and discontinuation dates to determine the life of a Mac is because most of the time those are the only published dates. It ain't perfect but it's reliable. I'm not sure why everyone gets so heated about it.