The link you yourself share features many comments, including by GitHub themselves, that this wasn't a new limit. They publicised the change earlier in the year.

As the product manager pointed out https://news.ycombinator.com/item?id=13229461 and as a Quora answer from June also reveals https://www.quora.com/What-are-bandwidth-and-traffic-limits-... the GitHub pages limits were public since at least June.