I'm not the dev but I finished a purist playthrough recently and I can remember only coring Sierra and Navarro. So I don't think that it's an option.

Is it possible to core territories even as the Reformers without it being a focus tree decision? The in game PipBoy guide says there's a way in the state screen, but I'm not seeing it so I'm guessing the Enclave is different?

@Puma ERX uses a different system in which you have to build civilian power grids in a state after completing the focus to use them in order to core states.

引用自 ᚛ᚉᚆᚐᚏᚐᚉ᚜：

Is it possible to core New Reno as purist?

It's been a long time since I've last played, and I vaguely remember being able to core them, but can't find any option to do so.

No. Though you can core Vault City if you choose the right focus.

引用自 Puma：

Is it possible to core territories even as the Reformers without it being a focus tree decision? The in game PipBoy guide says there's a way in the state screen, but I'm not seeing it so I'm guessing the Enclave is different?

No, ERB has it's own unique coring system as Reformers.