I'd argue this exercise takes more than 10 minutes if you want to do it well. The point is that the person from the past would find your ideas extremely fringe and absurd, so it would take a great effort to convince. In the same way that we now probably consider some moral views absurd that will be common in the future.

Would an effective altruist movement in the 1840s U.S. have been abolitionist? If we think we would have failed to stand up against slavery, what do we need to change, now, as a movement, to make sure we’re not getting similarly big things wrong? 

This, precisely this. I've thought this so many times as a response to dismissals of animal welfare concerns.

I think the concrete story actually makes the article more powerful. Communication in this space is sometimes too full of abstractions and lacking in concrete examples, which would make it more powerful for most people. And this is coming from someone who loves and studies abstract nonsense (maths).