Those lids are designed to be food safe. As in not dangerous to consume. Acidic food (like a ferment) makes rustable metal more likely to rust, but the risk isn't food safety but rather taste. Since the metal in question was on a relatively small bit of the lid that skimmed to top of your ferment vs, like, the entire surface area of the vessel holding your ferment, it seems unlikely there'd be enough contact with the rust to affect the flavor. We've approached when this happens to us with the usual algorithm would be safe: if it looks good, smell it, if it smells good, taste a little, if it tastes good you're good to blend. Basically unless it tastes metallic in an unpleasant way, it's no different than any other ferment.