Before we get any further, do you consider this device censorship? Honestly, I don’t see it as any different from controlling the media that I choose to purchase. Is that censorship?
So either this app is not censorship because it alters text that the purchaser is exposed to, or it is, in which case we are all ‘guilty’ of it, because you’ve equated choice with censorship.
(I’m not even going to go into whether controlling the media your children are exposed to is censorship. That tends to break down into ‘allowing your children to be exposed to material I find objectionable is negligence’ and ‘forbidding your children from being exposed to material I like is censorship’.)