Author here.
Our aim isn’t to try and line up with the calendar that was in use at the time, 2026 years ago.
Instead, the aim (for 28times) is to implement the rules of the proleptic Gregorian calendar[0] correctly. The bug was that those rules weren’t followed for some rare values.
But to address your comment properly, I think I should justify why on Earth someone would want to use the proleptic Gregorian calendar for year 0 and before. In short, because it’s a useful fiction.
The biggest reason is interoperability: a lot of software follows proleptic Gregorian rules, aligning around ISO 8601. Probably a clear majority of date/time software. We want to line up with all that existing software that assigns a specific meaning to a string like "0000-02-01". That’s the first of February in the proleptic Gregorian calendar, even though it was a different date in the ancient Romans’ calendar.
The other reason is to keep it simple: since most people use the Gregorian calendar in present times, it’s simplest (for us as implementors, but also for users) to extend its rules into the distant past and distant future. The alternative for be to have multiple sets of rules, e.g. Julian and Gregorian calendar. (You might say it would be even simpler to just not allow timestamps before 1582. But there are use cases, e.g. in astronomy or history, where people want to work with timestamps before 1582, and even before 1 AD.)
[0] https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar