The WP-Discourse plugin is designed to override those comments and replace them with replies from your Discourse instance. Since Discourse requires a login to use, you will have to choose if you want to use Discourse for discussions, or allow anonymous commenting in WordPress.
Notably, most users will only need to login into Discourse once, and that process is fairly simple. There are other benefits to engaging with users this way, but of course you will have to balance them with your goals.
I personally have not heard of this. Have you seen this elsewhere?
I would second what Maiki said, and add that the way I often explain this to folks is that you can think of Discourse comments in Wordpress as a way to entice people into your community. You likely want people to come across and spend time in your community itself. There are many wonderful and rich features of Discourse that will only ever be on the Discourse platform, even if two-way commenting (e.g. being able to compose a post in Wordpress) is ever possible.
That said, two-way commenting functionality has been discussed in more depth here: