I'm 80%+ sure the claim is BS

It's not BS for the people who don't understand the dark patters that guide users to enabling all of this stuff. That's everyone with a Windows PC who didn't bypass the Microsoft account requirement and went with all of the defaults in Microsoft Edge. Everyone using Chrome Enterprise/Education whose Google Workspace admins don't want to get into trouble for not backing up people's stuff (i.e., sharing it all with Google). Same goes for company Windows PCs set up with Microsoft Entra ID. It's everyone with an Android device and a Google account who wants their settings backed up or transfers to a new Android device. It's in the fine print and legalese for all of these products and services.