For example, deleted comments aren’t deleted, it just sets a deleted flag.
FWIW even when you properly delete something from a database table, the deleted row can be reconstructed from the audit tables. And even if that weren’t the case, databases are regularly backed up to tape drives or whatever - when people delete or munge all their comments, Reddit doesn’t go back into all the backups and make the same changes there. In fact, I would imagine that when they sell their shit to companies for AI training, they sell old pre-AI backups rather than a latest copy.