One of the annoying things about Google is that if it detects that at some point in the past you were logged in as a user it will not let you see Google Groups content without logging in (or going incognito).

There is no "just let me see the content, I can't be bothered to type my password right now" option, there is no "forget that I'm that user" option, the only option is to log in or to go incognito.

Logging in to Google Groups has been mandatory forever if you had logged in to a Google account before in that browser. If there are no traces of past login sessions (such as when using Incognito mode) then it opens just fine for unauthenticated users.

I also had to login with my google account to read it.