Backblaze don’t charge egress if you put it behind Cloudflare (even the free tier).
You won’t, however, be able to serve most e.g. media files or binaries this way, nor serve to clients like mobile apps, while staying within the bounds of Cloudflare’s terms for their free and “self-serve” tier paid accounts.
(Unless something’s changed since the last time I checked)
I dont think that would be right otherwise they R2 offering would be kinda useless. I feel the restriction was on video/streaming.
EDIT: OP is correct for CDN but if you use R2 even as a transparent copy from another S3 like provider it is allowed [1]