As it turns out the main Ohtotsuki clan knows about earth and all the boring drama happening in The Last. We know for a fact that Boruto's seal is shaped like the yin seal. I assume or rather predict that the seal infact is the yin seal just stored with ohtotsuki chakra. Maybe they've been spying on Tsunade and Sakura for quite some time...
only time will tell I'm afraid
only time will tell I'm afraid