I would argue that it's a reasonable limit on whichever freedom may be cited not to allow the creation and distribution of child pornography, even when it is fictitious.
That limit is probably where real harm was caused to a real child.
It's not implausible that deepfake child porn ultimately causes harm to children. Allowing people to repeatedly indulge in a weak fetish may reinforce it into a strong one, or blunt a disgust response which would prevent them from acting it out.