The word atheism comes from the centuries old French word athéisme which derives from the Greek 'atheos' which means without gods. It means disbelief in the existence of god or gods.
The word antitheism seems to have been used in more recent times. The prefix 'a' does not mean the same thing as the prefix 'anti' which means being opposed to something.
Therefore I agree with the majority that the word Antitheism is inappropriate in most circumstances.
Perhaps militant atheists could use the word.
I remember a recent discussion here on Militant Atheism. How do you distinguish it from anti-theism ? I probably come within the definition of both but I have never described myself this way. I don't use the words. Despite its definition the word 'militant' would make people think of guns and bombs while anti-theism sounds negative and too much like 'anti-christ'. The meaning of the words is not clear or generally understood like the word atheism. Communication is the name of the game. I would like to refer to my 'athéisme napoléoniennes' or call myself a Mushinronsha 無神論者 but no-one would understand me.