It appears to be related to sign-in flow[1], but may not affect signing in via Google.com or other Google-hosted services directly. It's the API for "Sign in with Google", far as I can tell here.
Thanks
For medium I recommend just blocking Javascript.
No signup popups, fast loads.
If you go to https://myaccount.google.com/permissions, you can turn off "Google Account sign-in prompts" on each of your Google Accounts and then you won't see any more of those prompts.
Don't know if Facebook has a similar option for their accounts though; I've seen a similar sign-in prompt elsewhere for Facebook.
I hope not because I'm doing it anyway. Medium.com makes me barf. Which is sad because it hosts some great content.