That's not a particularly good example, their latest self-promo post has a warning from the mod on it...

To nitpick: that's not a comment from a mod. Posts speaking from a position of mod will have [sysop] next to the title. It's part of a "hats system", which more generally allows people to speak as representative of something. IE if /u/johncheng was a Rust core dev, they can post whatever they want as John Cheng, but also write posts as [core dev] to say they're speaking as part of the core team.

It's a neat system but doesn't see much use in practice, aside from the occasional [sysop] warning.

Oh I didn’t know that! Thanks for the explanation.

That user's submissions are all within the past week. You may have found someone gaming the site.

I stopped posting three years ago, maybe the culture has changed?

Personally I don't see a problem with someone posting their own content, especially with voting based ranking. As long as it's not spam and is on-topic for the site, the origin shouldn't really matter. Compared with someone who posts only New York Times articles, I'd rather see someone post their blog entries.