You're using different words - the top comment only mentioned the reliability of the software, which is only tangentially related to the quality, goodness, or badness of the code used to write it.
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.