Where does the idea that a 24mm rim means a 24mm tire is optimal come from? Optimal for what?
Well, the general rule of thumb is that to optimize aerodynamics, performance, and comfort... You want a tire width to match the outside width of the rim. Hence, a HED C2 rim outside width of 23mm is best used with a 23mm tire. This is what HED claims.
So, it stands to reason that if what i have written above is true... then a tire width that is substantially more narrow than the outer rim width is not only not optimized aerodynamically and performance-wise, but it can be downright dangerous. You could potentially roll that clincher tire when cornering hard.
So.... Lets say you have a rim with an external rim width of 23mm, you would get the best performance from a 23mm tire. A tire a lot bigger, ex: 25mm, would give that tire shape a bulbous look and performance would suffer. Likewise, a tire a lot smaller, ex: 20mm, would actually be dangerous for fear of the rider rolling the tire in a corner. So look... Bottom line is this. A 23mm outer rim width is best used for a tire that is 23mm wide. Of course, a plus/minus difference of 1mm is probably ok. So in reality any tire between 22mm and 24mm is ok. However, I wouldn't drop down to a 20mm tire if I hade a wide 23mm outer rim width.