They most-likely used a Reddit Data Request. It’s kind of like Google Takeout if you’ve ever used that. If you’ve already deleted your reddit account it won’t work IIRC. I scrubbed my comments and posts with PowerDeleteSuite a day or two after I submitted the data request, but before I actually revived the data (took 20 days), but all of my comments and posts did show up in the data request. Don’t know if that points to reddit actually keeping that data in their database and just hiding it on the site or not, but either way, if it’s not visible on the site it becomes worthless to them unless they decide to provide that data to someone behind the scenes.