It's answered in the document: >IP Protection will use client authentication to limit the ability of bad actors to leverage the proxies to amplify attacks on s

It's answered in the document: >IP Protection will use client authentication to limit the ability of bad actors to leverage the proxies to amplify attacks on services on the Masked Domain List. Therefore, IP Protection will only be available to users that have been authenticated using the Google account they're signed in with in the Chrome browser prior to opening a new Incognito window.