It's against this ad
So back to the ad that sparked all of this, it sounds pretty innocent to me! I don't see anything about try to prove Emunah to your kid or try to change the seder and skip the yetzias mitzraiyim story and replace it with an Emunah session.
It was mentioned upthread several times that if the seder is done 'right' that will be the perfect transmission of emunah.
To me it looks like this ad is simply to educate parents on how to do a seder 'right'.
And the letter against it seems to be saying that emunah is a topic that shouldn't be spoken about...