I like to think of the aliens in this game as I do the aliens in X-Com: Apocalypse.
In that game, alien blobs from another dimension invade the dimension Earth is in because their universe is dying. The only way they can survive indefinitely in Earth’s dimension is by taking new hosts whose bodies are able to exist within it. As humans are all that’s left because TFTD’s conclusion basically killed off almost all life on Earth, the alien blobs basically try to take over by depositing themselves into humans and taking control of them.
The difference here as I see it is that the humans in this reality created the portals into their reality, possibly causing the eventual annihilation of their universe, so they either: a. See it as being only fair that we give them our world to make up for it, or; b. are simply non-sentinents that just exist, consume, and reproduce, and opening the portal allowed them in to do their business as usual. Sentinent or not, they’re aliens, and they don’t particularly need spaceships or the like when humans are more than happy to open doors for them to come straight to their doorsteps. Just another case of, “and man grew proud,” paving the way for human extinction.