The main problem this post complains about is that it's difficult to implement changes correctly, even as an expert. That's definitely not a good thing.
Adding on a new feature that is hard to understand is maybe a signal that the design does not support it.
There is a lot of mentioning of testing in this thread and in TFA. However, testing is not the main tool to achieve correctness. Testing is merely a sanity check. Correctness is achieved by being able to adequately reason about the logic of the code, making sure that all assumptions and guarantees are met.
TFA sounds like that reasoning is hard in the PostgreSQL code base. But it could also be that the author was approaching things in a too cavalier manner.