I'm really confused why I keep seeing arguments suggesting that that excerpt in any way matters to the argument of reparations.

I've never had a strong opinion on the topic in either direction but the argument of, as you put it, "this is _our_ wealth, which we created, and which we simply want returned to us." doesn't require excess wealth to have been created by slavery, it simply requires that some of the wealth didn't go to the people that did the work to produce it resulting in those people (and their descendants) not profiting from it.

Like I said in the other place I saw this, wage theft is illegal because someone isn't getting paid for work they did, its not in any way based on that theft providing economic growth compared to paying folks.