Marketing promises ARE a contract. Companies aren’t allowed to advertise a thing and then not do that thing. That’s false advertising and fraud. Companies aren’t allowed to say they offer a product or service for price X and then actually charge price Y. This is well established law.
You either didn’t read or didn’t understand the article. Multiple times in multiple ways the company said it’s offering a lifetime price, which is different than a price offered only for a limited term. They very explicitly said “T-Mobile will never change the price you pay” and “T-Mobile One customers keep their price until THEY decide to change it. T-Mobile will never change the price you pay” etc. etc.