On the product offering page for Free DDoS Web Protection, the features table shows that "Unmetered DDoS Protection" is available for everyone regardless of tier from Free all the way up to Enterprise. This change was rolled out on 2017-09-25, prior to this, there was a certain amount of throughput depending on price point (though, still very generous for the free tier from what I remembered).
Sometimes, people make up their mind about something and never update their knowledge, and it would appear this is one of those case here.