My guess is Google have got it wrong. But I think that the operative word is MAY.
Another point, I don't see how a sub-domain could/may be hacked, yet it not impact on the whole avast.com domain or forum.avast.com.
If you ignore the warning and visit the link, as this part (sub-domain) of the avast sub-domain is a restricted area and you are redirected to input your logon credentials.