I feel like partially because this is a "spiritual reboot" where they want to keep things simple for newbies, and partially because after getting burned pivoting the story repeatedly, they're being cagey, they didn't just confirm this, but at this point it definitely seems like the simplest answer is that the Endless are or are related to the Precursors? But admittedly that bumps up against other media (the recent novel Point of Light) that suggests a different arc for the Precursor's return.