I was curious so I searched the repo, the only mentions of this flag boil down to people being confused about there being two dates, and being okay with the current behavior.
The jj git backend happens to update only one of the two timestamps. There's not a lot of talk about the author date on the jj issue tracker.
I did not dig into the history of the code, so I can't say if it was an explicit design decision or just how it was implemented. Since there hasn't been a lot of demand to change anything here, I didn't see any active discussion about the choice.
