What is the reasoning behind the opinion of jumaah before dhuhr?

I know there are 2 opinions regarding the starting time of jumaah: 1. coincides with duhr and 2. before duhr. This Fatwa on islamqa kind of discusses both but in a biased direction. I am trying to understand about the second opinion, which is agreed by Ahmad ibn Hanbal and Ishaaq ibn Raahawayh. The islamqa link discusses the 'flaws' in their references and does not explain why Ibn Hanbal(or ibn Raahawayh) came to that conclusion in the first place. Also, what is the maximum extent before duhr can jumaah be offered?