The fact that Reddit can still tell you that the user deleted their account is proof that not all of that user’s data in their systems are deleted. It may just be a flag in an account that marks them as “deleted”, and so whenever data about that account is being retrieved, their API server will look at that flag, and tell the recipient that the account is “deleted”. People in the software industry calls this “soft deletion”.
I learned in SQL class that you never ever hard delete data when there is any alternative. On Facebook and Twitter you get a whole month to change your mind before your account can’t be recovered.