I think they're getting at something like this: If self-driving cars resulted in dramatically more miles-traveled-in-car per person they could be safer per-mile and more efficient per-mile while making some important total outcomes worse.
Relevant to this example: if people travel by car more because they care less about traffic when they're playing video games or on TikTok during it, instead of driving, overall congestion will likely go up which makes emergency services worse.
