Neither would they have to operate at artificially handicapped speeds, as no human operator would really be able to keep up regardless with the 10,000 or 100,000 cars running around.
Part of reacting and mitigating acute issues is being able to reroute traffic early enough, or slow cars down early enough, to minimize disruption (instead of a traffic slinky blocking everyone up).
There's no reason so far here that self-driving vehicles can't improve traffic and overall throughput, and it's a bold claim to declare L5 dead on arrival.