I can't reproduce this either. What I can reproduce is semi-logging in three accounts (I see them in the list but they don't have gmail, so they're grayed out).

In any case, I doubt the author tried this out. All multiple signon does is allow you to easily switched between accounts which you are logged in. Logging out of one doesn't affect the others, and changing the password of one doesn't change the password of any other account. It's basically a glorified switcher, and there's no vulnerability to speak of. I know this because I pseudo-linked the accounts, cleared all cookies and logged back in, and I was only linked in one account versus the three I had before.

It would be good if people reporting vulnerabilities actually tested them first :/

OK. It's true that I haven't tried this recently, but I did see this happen with one of my users a few weeks ago where I was suddenly in that user's personal email account on a machine they'd never used.

Hmm, this is odd. Clearing the cookies cleared all the signins for me. We'll have to wait and see, I guess (wait for a user with multiple Gmail accounts, I mean).