Try opening an incognito/private browser before going, and go directly to
atomio.slack.com. I tried from incognito and I don't see that page from there.
Also, whether or not that works, you should definitely submit an issue somewhere, maybe at the atom.io repo on github. I'm guessing those are atom's instant sign in options and they either don't realize what's happening or they don't have enough control over the slack API to change it. (Here's to hoping it's the former)