Neither, you’re reading it wrong. Think of it as codebases getting more reliable over time as they accumulate fixes and tests. (As opposed to, say, writing code in NodeJS versus C++)
I much prefer the alternative where it's written in a manner where you can almost prove it's bug free by comprehensively unit testing the parts.
Old software is typically more reliable, not because the developers were better or the software engineering targeted a higher reliability metric, but because it's been tested in the real world for years. Even more so if you consider a known bug to be "reliable" behavior: "Sure, it crashes when you enter an apostrophe in the name field, but everyone knows that, there's a sticky note taped to the receptionist's monitor so the new girl doesn't forget."
Maybe the new software has a more comprehensive automated testing framework - maybe it simply has tests, where the old software had none - but regardless of how accurate you make your mock objects, decades of end-to-end testing in the real world is hard to replace.
As an industrial controls engineer, when I walk up to a machine that's 30 years old but isn't working anymore, I'm looking for failed mechanical components. Some switch is worn out, a cable got crushed, a bearing is failing...it's not the code's fault. It's not even the CMOS battery failing and dropping memory this time, because we've had that problem 4 times already, we recognize it and have a procedure to prevent it happening again. The code didn't change spontaneously, it's solved the business problem for decades... Conversely, when I walk up to a newly commissioned machine that's only been on the floor for a month, the problem is probably something that hasn't ever been tried before and was missed in the test procedure.
And more often than not the issue is a local configuration issue, bad test data, a misunderstanding of what the code is supposed to do, not being aware of some alternate execution path or other pre/post processing that is running, some known issue that we've decided not to fix for some reason, etc. (And of course sometimes we do actually discover a completely new bug, but it's rare).
To be clear, there are certainly code quality issues present that make modifications to the code costly and risky. But the code itself is quite reliable, as most bugs have been found and fixed over the years. And a lot of the messy bits in the code are actually important usability enhancements that get bolted on after the fact in response to real-world user feedback.
Reality is management is often misaligned with proper software engineering craftsmanship at every org I've worked at except one, and that was because the top director who oversaw all of us was also a developer and he let our team lead direct us whichever way he wanted us to.
E.g. you might fix a bug by adding a hacky workaround in the code; better product, worse code.
The author is talking about the maturity of a project. Likewise, as AI technologies become more mature we will have more tools to use them in a safer and more reliable way.
New code is the source of new bugs. Whether that's an entirely new product, a new feature on an existing project, or refactoring.
Yes, I did that.