This message is just a “global” notification from Google to make sure you understand that potentially, by using the Google nodes available in KNIME AP, you can read & write to the various sheets available in your account. But of course you are in control of which sheets you read and write to by configuring those nodes yourself - the AP doesn’t read all of the data in your account by default, or anything nefarious like that.
Ultimately what KNIME has access to in your Google account will be dictated by how you set up your workflow. Does that help?