Tweets alone generate petabytes of data a year.
https://ankush-chavan.medium.com/twitter-data-storage-and-pr...
Also, many people would disagree that stuff required to run a business is "mostly unnecessary".
This was all already derived (correctly) in the original post. Recapitulating:
500m tweets/day * (conservatively) 512B/tweet * 365 days/yr ~= 90 TiB/yr
Assuming compression and variable-length encoding of this long tail in colder storage, it's more likely <20 TiB/yr (<=115B/tweet on average)
Yes, this excludes analytics metadata, which as you suggest would not support Twitter's current ad products. But your core repeated claim about tweets alone is two orders of magnitude off.
I wonder if the "Petabytes" figure being claimed includes pictures/videos that can be attached to a Tweet. In that case, I could easily see "Petabytes/year" be accurate.