Actually, when the stranger gives you her weapon ole dinklebot says "it's comprised of technology that should exist yet" it's a decent theory. There are a few others I've see. That run along this thought process.

Actually, there is a grimoire card that talks about the FWC's "device". The narrator of the card, seems to be talking to a ghost, and another person. There is a theory out there, that the device is how the stranger time travels/teleports. This, would actually explain how tthe stranger got the materials for her rifle. And, the line dinklebot says, still doesn't give evidence for this post. How did you make that correlation? Time travel is a pretty common thing in this game.

There are a lot of little suggestions through out the gameplay and cards that lead to an idea of the other races being, not from another galaxy, but from another dimension parallel to ours. The black garden missions being one "if this is the black garden, it's not on any map of known space and time" which means it could be in another dimension and time that has evolved differently than ours. Buuut it's also speculated that the black garden is actually INSIDE the traveler. That is based off of early concept art of what the traveler was supposed to be.