"You're signed in to the following Google Accounts. Add or remove accounts in Android settings," explained Google. There's also an option to "go incognito to browse privately or sign in temporarily". So now I have to add the account to my device if I want to sign in to a Google account in a mobile browser.
When tapping "add account", you're sent to a different section that lets you sign in to your account and enable various syncing options.
Chrome is just a browser, Google is just a site. Why would you add a Google account to your device when you only want to log in to that account in a browser?
To log in to a different account you can either open a new incognito tab or go to Chrome's settings, disable all accounts and dismiss the warnings. Or you can install a different browser. Why make things so complicated?
Update: There's a flag in chrome://flags called "Enable identity consistency between browser and cookie jar". When enabled, "the browser manages signing in and out of Google accounts". Google also tests a new profile management menu borrowed from Chrome OS. You can try it by enabling these flags: "Enable new profile management system" and "Enable the new avatar menu". "If you share a computer, click 'Switch person' to add your profile and get your own bookmarks, apps, and theme. Switching lets you keep your stuff separate," informs Google.