The Prius needs only nine horsepower to maintain 60 mph. The article I read many decades ago said it takes 16 HP to maintain 60 mph and that was on a full-size sedan (a yacht in today's size classification). It stands to reason if the motor can produce enough horsepower at 10% vs.13%, it'll use less gas. As for cost-per-mile, that's different these days with a $0.60/gallon difference.
Really? I'd truly like to see some documented evidence of 9HP at 60 mph! As far as "it stands to reason if the engine can produce enough horsepower at 10% vs. 13% it'll use less gas". Not necessarily true. Do a google of BSFC maps and look at one. BSFC numbers when mapped against RPM and load are islands similar to compressor efficiency maps. Lower RPMs do not mean best BSFC numbers.
Best BSFC is generally at peak torque. However, we don't normally drive around at 60 or 75 at peak torque which means we're using some sort of throttling mechanism. Whether that's done by closing the throttle body, inducing an Atkinson-style mechanism through use of variable camshaft timing, or inducing EGR the BSFC maps are all over the place. The manufacturer purposely uses certain gear ratios to get the best BSFC numbers You're also not considering auxiliaries- pumps (oil, water), valve train (higher power required at lower RPMs) losses.
So, I'm not sure why you decided an article you read "decades ago" before any of these innovations is relevant. Let's take a Tesla. Very aerodynamic and more so than the Prius, wouldn't you agree? Their 75Kwh battery pack is rated at 255 miles (they don't list speed) so we'll just pick 4 hours equating to 63.75 mph (close to 65). At that pace, it's consuming 18.75 Kw. That equates to over 25 hp. The ST is nowhere near that aerodynamic and has an entire thing called an automatic transmission (which the Tesla does not) sucking up valuable resources. See where I'm going?