Sure it could be more intelligent, but there would always be false positives. For example a Google employee's spouse could likely get correlated. False positives sounds like a bad idea for a system that tries to match personal and work accounts. Also in my case it would be harder possibly because I have a ton of personal Google accounts.
>A cookie shared between two accounts is even a stronger indication, as a person used the same browser to log in with both.
I never use the same browser for my work and personal accounts. But in theory that could run into the spousal problem as well for some people.
There is a ton of other information. E.g. for the “security” you are asked to provide a backup email address for your Google account. Also, for the same reason, everyone is now required to provide a cellphone number. If you used one of your Google accounts as a backup for another, or used the same recovery phone number for both, these two accounts can be linked to a same person. This comes in addition to other information about possible links, like geolocation or other trove of data they might have. If you added several accounts to check emails, your phone checks all of them every couple of minutes, from the same IP at the same time, whether you’re at home or not.
In short, the amount of data you are passing to Google, willingly or not, allows them to link these to a single person — you — with an extremely high certainty.
>A cookie shared between two accounts is even a stronger indication, as a person used the same browser to log in with both.
I never use the same browser for my work and personal accounts. But in theory that could run into the spousal problem as well for some people.