The source of this gamer theory is a tiny easter egg that fans located in Fallout 4. As you search for Prydwen, a Brotherhood of Steel airship floating above Boston, go to the main deck and find a room full of scientific experiments. There you will see a small garden of familiarly-looking plants.
The description of the plants states that they emit strange sounds and cause addiction. They also look very similar to the Nirnroot plants that can be found in Elder Scrolls V: Skyrim. So, does this mean that Skyrim and Fallout take place in the same world? That is absolutely possible, just in different time periods – one in the future and one in the past, but which is when?