When you go to the Gemini in Chrome section on the same privacy page it states:

'When you use the Gemini in Chrome feature, Gemini collects and processes page content and the URL from the browser tab you’re viewing by default. Some of the page content Gemini uses might not be visible to you."

I think they're being reasonably explicit about what they're doing.

Note: I'm in the EU so not sure if this is what's shown everywhere else.