Does the ‘Others’ folder in Facebook messages display communication only from those who have no mutual friends in common with user?
The facebook help answers this question: https://www.facebook.com/help/188872764494245/#What-is-the-"Other"-folder-in-my-messages?
Others acts like folders in your email provider.
Lets see if you want to have a clean inbox. when you read some message you just move it to others folder. this helps you have clean inbox and keeps messages.
You are right and if the user sends you a message who is not in your friend list for the first time it also appears in the others folder.
That helps in reducing spam somehow from the unknown persons on facebook.