So no, the domain is not a giveaway. The HTML code is.
and did you look into the whois?
it was registered by a company named Domains By Proxy, LLC. their slogan is "Your identity is nobody's business but ours."
that means they buy the domain for you and their name and company occurs in the whois records. for just a few bucks everybody can buy a domain from them and the whois record will look exactly the same as the two mentioned ones.
and to be honest: a real SEO expert wouldve known that
this is a way more sophisticated whois tool. these two show you whats behind the domains (physically):
1. diablo2hd.com is listed on a huge dns server that lists over 40million other domains, so nothing special about that so far, but nothing you would expect from a huge company like blizzard; playoverwatch on the other hand is listed on 4 different, small dns servers, hosted by amazon, not holding more than 13k domains. thats just a hint, not a real proof i guess
2. diablo2hd.com is hosted here: 188.8.131.52 - 3,533 other sites hosted on this server. do you really think that blizzard would just buy a generic webspace for about 3$/month to host its websites? no way i would say! and the playoverwatch.com whois confirms that, this domain is hosted on a dedicated server. thats more the style of a huge company like blizzard.
diablo2hd.com is probably hosted on an ordinary webspace which costs less than 5$ a month, not really blizzard-like.
playoverwatch.com is hosted on a dedicated server and probably runs in amazons ec2. i did not check that to be 100% sure but its strongly indicated by the facts. so this is more blizzard-like...