Should not be a problem here as the vacuum (if it's needed at all) would only be temporary to "make room" for high pressure helium. That would only be required if the helium had to be of particularly high purity and gp wasn't at all implying that this is the case, only that it could be a challenge if it was. From my uninformed perspective, it's just as likely that impurities would condense at the bottom and rest there happily ever after as they would be thrown into an permanent phase change cycle that rims efficiency even in trace amounts. The best case scenario I'd like to read is "actually we don't really need helium but it gives a slight performance edge over other gases and we decided to launch with the setup that gives us the best numbers". No idea if that's close to or very far from reality.