Even if it's not perfect, it's a significant step towards removing discoverability. The Google crawling will eventually drop the content associated with the username, making it massively more difficult for someone to dig up past history.

A motivated individual could still dig things up, but many people (even most users of reddit) are unfamiliar with the archives out there.

The issue is that regardless of Google/Bing caches, there are sites like Pushbullet that archive ALL comments and posts that were made on Reddit, with APIs that savvy people can use to check deleted/edited comments. Awareness of these archives seems to be spreading through Reddit.

There are quite a few HN mirrors out there, too

https://hacker-news-example.factor.dev/

https://hn.matthewblode.com/

https://nilaykhandelwal.com

https://innerself-hn.com/

I'm not really sure why people would do this, except perhaps some seo shenanigans?