I’m going from memory here, but I don’t think the edited comments.php file is the cause of the issue. What should be happening is that instead of your theme’s comments.php file being loaded, the WP Discourse plugin should be loading its own comments section when this function is called from your theme: comments_template().
Have a look at your theme’s single.php file, or whatever file is used for displaying a post that you have linked to Discourse. See if the file contains a call to comments_template(). It will look something like this:
if ( comments_open() || get_comments_number() ) :
comments_template();
endif;
It’s the call to coments_template() that should trigger the WP Discourse comments to load.
I could be wrong about this. I can have a closer look if that doesn’t help to sort things out.