Fuinjutsu need chakra in order to activate. Meaning they can't release it if there points are closed.People keep failing to realize what they are typing, calling it Tsunade/Sakura's own STORED chakra is the POINT.
It is not present in their body at the point of them having their chakra points sealed if that happened. IT is locked away in a S ranked seal on their forehead. When it comes forth the physical markings represent the flow of chakra along the body pumping chakra BACK into the body because it was NEVER there.
It doesn't have to be a chakra being living inside of you it just has to be another chakra source. And Junchuriki and Tsunade/Sakura are the only shinobi who have existing two separate chakra sources.