Before the M3 migration, the doc made sense. but after, it needed to specify which material version does this issue occur in.
The _AppBarDefaultsM3 don't take the brightness in consideration anymore:

<img width="517" alt="image" src="https://github.com/flutter/flutter/assets/124896814/d4dbe278-5b50-42d7-9143-c54d343efddf">