The value
- timeline logic applied loosely; Oak = same era hint
Where it comes from
Junichi Masuda Comments On The Pokemon Series' Complicated Timeline Nintendo Soup · 24 October 2019 Open the original ↗
The line in the source
It starts to get a little complicated if you pay too much attention to timelines.
In English
Masuda (via Nintendo Soup, secondary; original GI page for this answer not located) says timelines are deliberately not enforced.