"a system used to help Google comply with search warrants" reads to me like it was something google used when notified the warrant was issued. That's not the same as law enforcement agencies/officers having access themselves which is how I read "designed to give law enforcement access to people's emails".
At least in this case, I wouldn't call it a "loophole" exploited by hackers. It's just a system they have to make it easier for them to provide information under warrant. If the information exists (and presumably it does, it sounds like they would have liked the contents of the emails), it's possible a hacker might get it, even if they don't have a "quicky warrant" front end but rely on a more manual process. Details are sketchy so who knows, but maybe this unintentionally turned out to be a form of honeypot where they got limited (subject lines, meta data, I guess whatever you must provide to LEO) information out of this system, whereas had it not been found they could have penetrated something else, perhaps better protected, but might have held even more information of value to the attackers.