I’ll assume add_css and add_js are wrappers for wp_enqueue_style and wp_enqueue_script.
As such, the way you register any style / script is through either wp_dequeue_style and wp_dequeue_script, where you pass the handle of the scripts.
See: https://developer.wordpress.org/reference/functions/wp_dequeue_script/
Now, from that code alone, it doesn’t seem like he’s passing any handles to these “register” functions, so take a look at the source, he has to pass their names within that class’ static methods.