Ah, yes, good catch.
To be more precise: NotePlan needs to be open on the Mac to allow “syncing” between Obsidian and NotePlan. NotePlan itself syncs between devices fine. So, you don’t need NotePlan open on a Mac if you’re going to be switching between iPad and iPhone, but you do if you’re switching between NotePlan on iPhone and Obsidian on a Mac.
I’ll add this to the guide, thanks!