As their true host, Naruto could easily access their chakra as he did vs kaguya. That was the purpose of the sage stating that, he told kurama to remain with Naruto to help manage the meetings. If Naruto was in an emergency He would simply request a psyche meeting asking to borrow their chakra.Actually he said that the bijuu could talk to each other through Naruto and u do realize that this become one thing happened before Kaguya meaning it was just Nardo becoming jinchuuriki.
Hagoromo never said anything about Naruto siphoning chakra when need be :|