No Multiplayer would/will be a mistake
I read the FAQ guide that says that "Agent 47 works alone" for the "Will there be Multiplayer?" question. This doesn't necessarily rule out multiplayer, as it's not a "No."
Assassins Creed III will be released before Hitman: Absolution, and chances are it will have it's Multiplayer included after introducing it in Brotherhood and continuing it in Revelations. Whether it's fair or not, these games will be compared continuously after they both release.
If they want this game to be as successful as it can, they should include some form of multiplayer, similar to Assassin's Creed. Assassins Creed was able to implement multiplayer into a singleplayer-heavy game, and it was definitely fun as a change of pace from the singleplayer - Hitman can do it too.
It will be a mistake not to include it, from a "maximum amount of fun" standpoint, and from a fiscal standpoint.