I’m not sure that applies here. Generally, when measuring something, you use less. Like I wouldn’t say , I just drank from my glass and it now has fewer waters in it. In this case, “natural ingredients” is a set of things that are being measured as a single “ingredient”. Like let’s say the natural ingredients are soot and berry juice. Would you say the paint has fewer or less soot and berry juice?

But then again language is all made up, the rules don’t matter, and you’re only truly wrong if the meaning is lost.