This does not change map generation, beyond generating more starting tiles using normal/existing rules (without modifying their design/pattern). If they would have had biter bases when you explored them manually, they should be identical to how the mod expands the map.
Besides asking the game to generate a larger starting area, it has no impact on chunks.
After starting a new game, the mod only reads newly generated chunks for resources, and if found, tags them.
It cannot tell before a chunk is generated if it will have a resource or not. It is designed to look at a chunk the tick after it was generated to see what the game added, and tag accordingly.
Most of the behavior can be found in Resource-Marker / control.lua if you are interested in diving into the deep end. 👩💻
Hope that answers your questions -- have fun! 😀