We both immediately cracked up laughing.
If it had said "war is a lie" or "war is lie" or "the war is lies" or something, it wouldn't have been funny, but because it was precisely a line from Portal but with 'cake' replaced with 'war', it was exceedingly funny.
It made me want to put a sign on the next overpass that said, "War is cake."
I'm sure it was not intentional, and I don't mean to belittle the sentiment of their message. It just stuck a chord other than the one they intended.