Getting 100k people to download 100GB seems like a rather lofty goal. Has this ever been done via social media? The only case I can think of is a game like Call of Duty, which has a ginormous marketing campaign behind it. Even so the download wasn't linked via social media.
Most people don't even have 100k followers. The follower-to-click ratio isn't going to be 100% in any situation. Click-to-full-download ratio is going to be low too, especially so at 100GB. A lot of people don't have that much free space on their devices even!
I think this scenario is firmly in outlier land, and thus not really relevant to cloudflare's calculations.
What's the catch? Imagine a few cases. Let's assume s3 volume rate of $50/TB.
-I post 1gb video file on Reddit. 100k downloads / month: $5k
-I make a 1gb desktop app. I have 100k downloads / month: $5k
-I post 100gb data file on Github. 10K downloads / month: $50k.
Would I pay $0 on R2? And would there be throttling / rate-limiting?
[Edit: Added more realistic examples]