This demo is very easily learnable as there is only one action which you can do, which is drag the squares on the sides, and since the squares are big and have feedback on hover, it is easy for the user to find this action.
It is also efficient since in the common case of when radii add up to 100% you get to desired shape in four click and drags, but they also made the uncommon case of any border radii possible through the 'full control' button.
For safety, the website allows you to change the height/width to arbitrarily high numbers, which is not very safe.
This demo is good in terms of usability, as it is very simple design and it is clear what actions you can do. However, I think it does not do a good job of teaching the CSS since the website does not explain what the clamp function is and how the second argument of it is calculated, so user of this demo would not be able to apply this function on their own.