Actually thats not entirely how it is viewed.
We do not, nor have not, ever in the past, and won't in the future believe that everything every different church teaches is wrong. Thats what you are making it sound like, and thats incorrect.
We believe many have lots of good things, and Eternal Truths in them. We believe because of the divisions in the churches, some have this, other might have that, and some have something else thats all good.
None have the whole picture though, which is what Joseph Smith did. He had it all put together for him by God.
He made the claim (or said the angel/Personage told him) that NONE were good; not me.
Has the LDS organization EVER published a rebuttal of this claim?