GitHub has a soft limit of like 100 GB/month on transfers for Pages. According to the Adguard blog post traffic was already several TBs a day before the issue arose.
'Legitimate' users of the list would clone/pull the repo to their own mirror?
EasyList updates frequently, many times each day, as the commits to that repo demonstrate.
I suspect the latter. I don't know how to make a repo public but limit web traffic to it. Do you?