His bug allowed him to inject links into verification emails sent by Google Scholar. He claimed that he could inject CSS links too, but that didn't make this problem any worse. Why? Because it's up to mail clients to load the linked CSS stylesheets or not. Gmail, for example, would never load those remote CSS files. If your webmail client does that, it's time to switch to a better one.
So he could inject links, which is annoying, but still a very minor issue. It may make phishing a bit easier, but you know what phishing has always worked against average Joe if you try hard enough. That means that this problem doesn't really give an attacker any advantages that he couldn't do by himself.
Disclaimer: I'm a member of the team that handles VRP.