> Did someone not understand what went in?
Unlikely, from the context.
> Might someone have offloaded their thinking to some external tool?
No, according to the author, since the features predate most current LLM assisted coding.
> Did people stop caring?
I'm not sure what this means. Presumably things were reverted because people cared enough to make sure it was good quality before being released.
> Did someone not plan or design a feature in full?
An admitted possibility
> Were the timelines unrealistic?
Depending on what you mean, that may be correct according to the article, but I suspect not in the way you mean.
> Maybe with so many reverts timelines should be revised?
It sounds like there was a very large increase in load in some of the people that would commonly review during this time period, and that may or may not continue to happen to a greater or lesser degree in the future, it depends on how LLMs advance in finding bugs and if there are other large waved of bugs reported.