"Jia Tan" was not a contributor, but a maintainer of the project. The key point here is that this is a multi-year project of infiltrating the xz project and gaining commit access.

In a large tech company (including ones I have worked at), sometimes you have policy where every change has to be code reviewed by another person. This kind of stuff isn't possible when the whole project only has 1-2 maintainers. Who's going to review your change other than yourself? This is the whole problem of OSS right now that a lot of people are bringing up.

I maintain a widely used open source project myself. I would love it if I could get high quality code review for my commits similar to my last workplace lol, but very very few people are willing to roll up their sleeves like that and work for free. Most people would just go to the Releases page and download your software instead.