Using Symbol manager with v.47, works well just as mentioned in this video - https://www.youtube.com/watch?v=bkBTQHVpups, but may cause some troubles with symbol names when moved from one folder to another.

Now the site should work well, so yes, it's a UI Kit built with smart components made using nested symbols and Sketch app best practices.

When working with a new project, it might take you an hour to set up your file, and this process occurs in design life very often. So in terms of using a pre designed styles and components that only need a brief customization can save your time as well as money.

Thank you for suggesting, a bug was found and fixed, now the site should work well.