The same component (previously called M2, later just known as mail) is used to process feeds and mail. If you have no mail accounts configured, the sidebar (which will be titled Mail) should show a list of all your feeds, while the main tab (which will be named Feeds) will show a list of all your messages along with the text of the highlighted message. If you want to read only one specific feed, you can click on it in the sidebar - the name of the tab will change to the name of that feed, and only messages from that feed will be displayed.
I understand the confusion, but the mail component is designed to work with the sidebar displayed like that. You can press F4 to hide it, but it will reappear whenever the mail (Feeds) tab is closed and then re-opened, or even if you focus a different tab and then go back to the Feeds tab.