Tech companies usually take backups y'know? There's almost certainly older snapshots from before the AI craze, or at the first sign of trouble ...
The main buyer is Google. Google actually ALREADY has all the data on Reddit, they scraped and cached it all long ago. Remember how Google used to offer mirrors of pretty much any site? Well, guess what, they probably will have all that valuable gold stashed. So it's not like reddit is actually transferring terabytes to Google, it's just a licensing deal, and the execs are having a fun chuckle about users trying to "delete their data"