If its not true then why do many cars with self-driving build it in as a feature to allow breaking the speed limit (up to a limit, in my car it still won't go above 15mph above the limit) in order to more closely match the flow of traffic.

I think theres a bit of nuance. If the flow of traffic was all being crazy lunatics then yes I'd say so. But I would feel safer without other cars flying past me/around me because I decide to go 70mph when everyone else around you is going 80-85.

I speed on the freeway too, I just don't believe it's "safer". The margin between the traffic speed and the speed limit is just not generally large enough to be a hazard, and driving at a higher speed is just more risky and potentially lethal in every way.