Actually, you're right. How would this work? Is Cloudflare really willing to foot the bill of 20 TB of bandwidth per day for a small text file that costs $0 to store?
Though, adblocking is a big business, many actors there are getting large revenue.
For example, Eyeo's income was 50 million USD per year last time I checked (and I guess most of it is actually profit), so they can find a solution if they really want.
https://img.phantasmagoria.me/img/96XJrjejoHNdrQv7.jpg
Even if you have a private bucket, you can give people a signed link with read access, for up to two weeks, IIRC.
But it'll still cost them money by number of reads