Is the example code in the connectors guide is for Sapper?

The XDN Connectors Guide example code for dev.js, build.js, and prod.js all reference sapper.

Is the init.js for sapper as well?

Yes, the init.js example is also from our implementation of Sapper connector. But all the other frameworks are also implemented through the public Connector API and Sapper was used just as an example.