Why isn’t Ordinary Jo a Humean or an Aristotelian? Because Ordinary Jo thinks akrasia happens all the time, and Ordinary Jo thinks if knowing what’s right and wrong were not a different kind of state from the wanting or desiring which actually affects your action, akrasia wouldn’t be possible.
Why does Ordinary Jo think that? Because Ordinary Jo thinks that the same person can’t possibly have competing desires or contradictory beliefs at the same time. And if you think about it, the Humean and the Aristotelian only allow akrasia to be possible if the same person can have competing desires or contradictory beliefs at the same time.
You seem to have an awkward choice between these two options:
Akrasia is about the relationship between different states of mind within the same person. Disagreement is about the relationship between states of mind of the same kind within different people. Now imagine we have a moral disagreement, where I say ‘That’s wrong’, and you say ‘No, it’s OK’.
And now suppose we’re Humeans. That is, we believe that thinking something is right is a bit like wanting it to be done, and thinking something is wrong is a bit like wanting it not to be done. Along with thinking that desires aren’t true or false, or right or wrong in anything like the way that thoughts and beliefs are
Are we really disagreeing when we have that moral ‘disagreement’? It doesn’t look like it, because it doesn’t look as if I’m contradicting what you say: I’m just expressing my desires, and you’re just expressing yours. So it looks as if there’s a connection between taking akrasia to be genuinely irrational and thinking moral disagreements are genuine disagreements.