That picture has been around for a long time and when I initially saw it, I had a difficult time believing the numbers on it. Although after I thought about it, it really isn't that unbelievable. Mower technology is in a completely different league now with the quality and consistency of cut. For example back then a greens mower may have had 7 blades, whereas now 14 is common, so it is essentially double cutting as it travels and that's before considering that the reel speeds are greater now too. The mowers now are heavier so you get more roll, groomers & brushes are available to reduce grain, we have powered rollers and they are just some examples on maintaining the surface. The turf varieties now are infinitely more dense and bred to mow at heights that could not be believed back then, and advances in aeration equipment, irrigation, greens construction etc all contribute to the turf being able to withstand lower heights and more aggressive management. I'm going to guess that Harbor Towns lower number is probably due to it being bermudagrass - a quick search says that #2 used bentgrass back then, which would explain why it was higher.
Last year I wrote an article about the stimpmeter and the quality of the information it gives (its the last section of the link below). The issue is that all the stimpmeter really measures is distance - which isn't speed. It is really a device for measuring consistency across a course (which incidentally was why it was designed in the first place) but not different courses - as there are too many variables for it to be consistent from property to property. Rollout should really be the measurement as it is the perceived version of speed. Personally I think it is how the ball travels off the stimpmeter ie on shorter turf it bounces and accelerates as it hits the turf so is travelling faster when it loses momentum and starts to slow down, making it appear to be gradually slowing from a higher speed - think of it as a bell curve of acceleration - than on longer turf, where the tip of the stimpmeter sits into the canopy a little so there is no bounce and the ball is constantly deaccelerating from when it hits the turf, although it travels the same overall distance due to the initial force from the roll down the stimpmeter. Think of it as a linear deacceleration where the perception is that it didn't "rollout" as it just consistently rolled to a stop. I'm no physicist so am probably way off, but the fact you can have the same stimpmeter reading but a very different "rollout" highlights its biggest flaw. A device like a pinball launcher would be better it would be a linear force applied to the ball, although the USGAs new ball looks like it might be the solution as its gyroscopes can measure the acceleration and speed as it rolls.
https://www.golfdom.com/musings-from-the-ledge-mr-stimpy-isnt-a-liar-but/