I don't think he tossed it out completely without comment. He simply pointed to the graph and said (or at least implied) "yeah you can keep decreasing it if you drop speed further, but this is the point at which it seems the return on investment starts significantly decreasing".

I get the impression from your comment that this is the first NJB video you've come across. It might seem strange in that context. But this is one piece in a large history of discussing what makes good road safety, urban infrastructure, and city planning. For example, one thing that he didn't really discuss in this video but has mentioned many times before is how the most effective way to slow down drivers is actually not just lowering the speed limit, but changing the design of the road to make drivers feel unsafe driving at higher speeds. Things like objects near the side of the road, narrowing the road, making it less straight, making corners at intersections sharp right angles rather than smooth curves, etc. And this video doesn't talk at all about the importance of infrastructure that encourages cycling outside of merely lower speed cars.

The "goal" is...complicated. Because there are a whole bunch of different factors that reinforce each other in a virtuous way. Safety, happiness, health, economics. Good urban design is good for all of them and more. Even just lowering the speed limit increases the safety for cyclists, which increases the rate of cycling, which is good for health, environment, government budget, and small businesses' bottom lines.