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The critical words in the statement are not "We will not scan or read your Gmail messages"; they are "to show you ads."

In other words, they reserve the right to scan or read your Gmail for any purpose other than showing you ads. So they can still read your email for things like creating that purchases list, as well as a myriad of other tasks. As long as that task doesn't involve showing you an ad, your Gmail is wide open to them.

The list of purchases is the feature.

It's used for Assistant