It's written this way to be compatible with the actual released e107 2.2.1 version. But they are already preparing changes in the core that will change everything (HTML templates, psst :}
This is the reason for separate development repository.
The main purpose of this theme is mainly to test all templates with bootstrap 4.
The base of this theme are templates and snippets by startbootstrap, mainly this one: https://startbootstrap.com/templates/modern-business/
A lot of work before us, mainly because I will rewrite all my bootstrap4 theme to this standard.