They said exactly how they did it, it's right there in your quote: when someone reloads a page running on their dev server, the server checks if the CS file has

They said exactly how they did it, it's right there in your quote: when someone reloads a page running on their dev server, the server checks if the CS file has been modified and triggers the compilation… So no, it's not a build system feature, it's a server feature.