"...pulled up the person's email account...[and] a list of other Gmail addresses that the friend had registered but didn't think were linked to their main account—within seconds"
This surprised me too. In the absence of any further comment from Google, I'd be very interested to see some journalists doing some investigation here.
Assuming that this is real and not mis-reporting or user error, I'm guessing Google links using either their google.com cookie, IP address and/or browser identification. Any of those methods have potential for errors (in particular they mean you should never share a browser with another person in case your account ends up linked. That seems.... extreme..)