From that article: "Since AT&T no longer sells unlimited data plans to new customers, these policies affect only longtime customers with grandfathered plans."
This is for people who were on long-term or never-ending contracts, and those contracts are no longer available. Giffgaff doesn't do contracts, and even if a goodybag is seen as a contract it is only for 1 month so giffgaff have the right to refuse to sell it to you again.
If AT&T have managed to get the number of people on those contracts down to a manageable level, they can probably afford to give a more generous allowance to those few people.
If it is only a very few people, it will be a good publicity stunt.