I feel like if you're modifying the signature after they sign it and approve it - that could be a problem. As long as the modifications are applied in real-time (or with explicit user confirmation after modification), I think it is morally okay (and probably legally, but I'm not a lawyer).
You may find users who get mad if your settings are too aggressive though, and if they're unable to get a signature that they approve of.
Given it a bit of thought (as I was also puzzled), I think the comment was about the modification compared to a normal way of doing it. So if I would normally write a scribble with angular lines for a signature, and it might be consistent across different places, this brush could smooth it out and modify what it might be.