I believe they mean: Even if you don't use gmail, but you correspond with other people who do use gmail, the other people's copies of your messages are linked t

I believe they mean: Even if you don't use gmail, but you correspond with other people who do use gmail, the other people's copies of your messages are linked to you and handed over. It's hard to avoid corresponding with other people who use gmail. But it's even harder, when you realise this includes people using their own private email domains that are handled by the gmail service without your knowledge. Many companies do this, even if it's just for the excellent spam filtering.