Since this chapter was the Biggest Asspull, let me try to be sincere and be generous to damage control
First of all Isn't Susanoo a jutsu of the MS? Heck Itachi himself stated when he used it that this was his final technique.
And Technique = Jutsu in Naruto right?
What happened here is just a Mangekyo copied another Mangekyo's technique(jutsu). We don't know the full extent of the copy abilities of the MS because all who have it always used the techniques that comes with it.
That been said Kakashi's Susanoo may have the ability to copy other Susanoo "abilities" like he did with Sasuke's Susanoo's ability to fly. Remember that each Susanoo in the series had a special abilities that come with it:
1. Itachi = Able to seal away soul's with his Susanoo's sword.
2. Sasuke = Flying ability
3. Madara = Pure destructive force
4. Kakashi = Able to copy the special abilities of other Susanoo like he did with regular ninja that he was famous for.
As you can see with these examples that each Susanoo is unique on it's own based on the user using it.
As to how he's able to use such enormous chakra, Obito physically and merged Kakashi to give him Chakra, thus having chakra reserve, and it's only "temporary"