Quite simply they are not in the same universe because of the mythology of the Elder Scrolls and the setting of the Fallout games. Fallout is set on Earth, plain and simple. A post-apocalyptic Earth with everything from psychotic killers to mutated humans and a few megalomaniacs thrown in. Elder Scrolls is set in a bubble universe, more or less. It was created eons ago, by this worlds standards, when a god-like being was killed by another god-like being. In the process the world of Nirn was created. Roughly. There really is next to nothing outside the world of Nirn, the stars are not the same thing that we see when we look up into the night skies. In that universe "gods" are very real in that they communicate with the mortal folk. Yes, they actually talk to them. Mostly it's the ones considered bad, but still.
All that this plant in Fallout 4 is is just a little easter egg much like in Wolfenstein: The Old Blood, in the very beginning of the game you can find a helmet that is a replica of the iron helmet seen on the cover of Skyrim. Same company so they thought they would throw in some little things for the fans of other series.