The argument doesn't change it's still a drink made by Sister they had inside the ship and there's nothing to stop her drinking it offscreen.
It could also be that by absorbing the chronophage he can now survive in space but I doubt it because Mashima uses easy solutions.
In any case there's no...
Surely in the next two chapters they will prevent the earth from being destroyed again and at that point everything will be solved, that's all that's missing and shiki's plan is complete.