Blizzard can't trademark Mephisto, it doesn't have to be them. It would also be extremely bizzare to use an actual character name in their universe as a "secret" codename. So I'm going with coincidence.
Mephisto would be an odd choice for HotS though, since he hasn't been relevant in a very long time. That would almost confirms a D2 remaster is going on, though you'd think Baal would be the better choice there. I can't find anything remotely credible about this rumor though anyway.