I had this same problem!! I was trying to set this up thru Google Home. But when I downloaded the HBO GO app and turned off my mobile data and just used my Xfinity wifi, I opened the HBO GO app and then found the provider list to be incredibly longer and Xfinityl was actually listed this time!! I was able to log in this way to HBO GO. But by trying to set up HBO GO on Google Home with my mobile data on it seemed that XFinity not Comcast would appear in the short provider list. I don't know why this is. I hope this fixes your problem as it fixed mine!
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