The report claims that Facebook appears to be making some changes from the server-side of things, meaning that these changes won’t need you to update the app in order to see them. One of those changes is multiple account support, as you can see in the screenshot above in which it seems to suggest that users will be able to add more than one Facebook account to Messenger.
For the regular user, multiple accounts on Facebook seems unnecessary, but there are people out there who do own multiple accounts. For example some people might have jobs as social media managers and might have to juggle a variety of accounts for numerous brands or people, and have to be able to reply messages from different accounts.
Some Facebook users might also be popular enough where they end up creating multiple accounts because they have hit their friend limit. That being said, these are server-side changes that appear to be in testing at the moment, so if you don’t see it, you probably won’t until Facebook decides to turn it on for everyone.