At the top of long discussions, there is a list of users who posted in it.
When you click a user avatar, it reveals a user card, in which you can click a nice “X posts in topic” filter button, that will show you all their posts, without the other user posts.
But for some users, the user card only shows “This user’s public profile is hidden.”
I guess they have activated some kind of privacy option.
But then in this case, the “X posts in topic” filter button is also not shown, which I think is not by design.