The delete feature seems to have been implemented with the idea of "poster's remorse". It's sufficient for people who want to take back one or two posts they've recently made. It's total shit for mass deletion. Whether that's intentional or lack of foresight, well, who knows with Reddit?
Since it can't be said often enough: former redditors, Power Shift Delete isn't going to work on your older comments (Reddit only indexes the most recent 1000) or anything in a sub that's gone private. You want to get everything, you're gonna need to pull a list from a GDPR file or from a torrent of a Reddit archive. I strongly recommend that people delete their posts as best they can but not delete their accounts until they're sure they've gotten everything they want to wipe.