An ABI matters here because it means that the Xscreensaver component would have to stay in step with the compiler component, which means you've created an extra

An ABI matters here because it means that the Xscreensaver component would have to stay in step with the compiler component, which means you've created an extra build time dependency. It also means that if a user had their own Xscreensaver module, it too would be required to be using the same ABI. In short, there's no particularly compelling advantage and a lot of disadvantages for a widely used and distributed component where you want to maximize its availability on platforms. C++ is just a tool; it's not always the right one. The maintainer has made implementation choices to maximize compatibility and portability. You have to remember that Xscreensaver is designed to be used and executed on a wide variety of platforms, some of which are relatively old.