That's actually a step in bootstrapping the system. Generally, to bootstrap a language a new language involves building a minimal bootstrap compiler/runtime that is just enough to be able to rebuild the full system:

1. build bootstrap system with existing tools 2. compile full system with bootstrap 3. compile full system again with new compiler

Or something like that.

Edit/Note: The OP linked document is a pretty good explaination of the whole process as well as the implications ...