There is a difference between discussions in the feel, be it the site feed or the document feed, and discussions viewed in the Discussions Panel.

The purpose of showing discussions in an activity feed is to inform users of the latest conversations happening on a specific document or site. These discussions do not need a precise hierarchical order, only a timeline of when they occurred.

However, in a Discussions Panel, which is dedicated to starting new discussions and replying to specific conversations, the organization must be different. These discussions cannot simply be organized by a timeline of the latest activity. Instead, they require a hierarchical order: showing which message is a reply to another, and how many replies a single discussion has.

Because of this, it is logical for the Discussions Panel to prioritize the thread structure and hierarchy of comments, with the timeline being a secondary consideration. This differs from the main feed, which is organized primarily by the chronological order of events.

The way this could be adapted to look similar to the feed is by displaying only two levels of a thread initially, allowing the user to zoom in to view the entire conversation.