That is to say, it would have sounded more natural to some people.
Adverbs are typically gradable and can take comparison.
That means that they can be viewed on a scale of less or more. Their intensifiers modify them by showing how far along they are on the established scale. The comparative forms can be used as both adjectives and adverbs. At this point they are almost interchangeable. Farther , say grammarians, is on the way out.