I cannot imagine a Smart car being stable over about 50 mph with the short wheelbase it has.
I’m not sure if he couldn’t go at a speed keeping up with the flow of traffic at 65 MPH or that he just chose not to do so (i.e. some greenie idiot who thinks that driving slow saves fuel and thus “saves the planet” even if it impedes the flow of traffic and creates a dangerous condition as other drivers try to pass), but yes, to me it didn’t look like a vehicle that I would want to dive or be in, on a major interstate highway.