But your earlier point is also true: it's a serious game and you wouldn't expect it.
Dude, Stronghold is not really a serious game. There's a lot of comedic content in there.
Now a WTF Christmas easter egg can be found in Operation Flashpoint (probably also ARMA:CWA, never checked in ARMA 1 or 2). I got the GOTY edition for Christmas a long time ago. I installed it right away and WTF, many trees were turned into christmas trees. Now THAT was fucked up. I was almost sad when the Christmas was over and they disappeared. :D