Reading through the linked paper and other resources, I don’t think concrete security issues with face blurring have actually been identified, especially with the pixelate filter (as opposed to gaussian blur) present in this app.
If we want, I’d be fine with amending the existing warning to say:
Blurring is not as secure as simply drawing an opaque box (i.e. a black box) over a portion of an image. You should especially never use blur to redact text in images. For this, we suggest apps like Pocket Paint.
…but not really to say anything about faces specifically.