Extraterrestrial can be justified
Your ambiguity in "extraterrestrial" hinges in your definition of Terra. Right now, we have one planet, which is why it shares the word for "ground" (earth vs Earth, terra vs Terra). This applies to many things. We wouldn't be calling our sun "the Sun" if we had more than one sun (i.e. because we inhabit multiple solar systems or live in a binary solar system). The same applies to the Moon. We would've given them a proper name (different from "moon") if there was a need to distinguish between them.
When the colonization process begins, i.e. when have more than one thing (planet, solar system, ...), a need to distinguish between them arises, and people will naturally find unique names for the unique things.
The question is, how does the definition of the non-capitalized word change when there's more than one inhabited planet? Does earth refer to any kind of soil, or specifically soil from Earth? I think you have freedom of choice here.
However, I can reasonably argue that the Terra in extraterrestrial is always going to be relative to the speaker. So the question is: is the speaker speaking as a citizen of NewPlanet, or is he speaking as a citizen of the entire human civilization (or a subset of humanity that defines itself independently and inhabits several planets).
Depending on what the speaker defines as "our domain", extraterrestial refers to "from outside our domain".
This means you can do what you want with it. You could keep the definition to be "from a non-human-inhabited planet", or you could put the word in the mouth of someone who sees everyone not from their planet as an "extraterrestrial" (heavy analogy towards the use of "alien" to mean non-citizen even in today's world).
The short answer here is that the conflict/ambiguity that you currently see with your 2023 eyes is likely something that will have been addressed during the colonization of additional planets, so you can decide exactly how the meaning of the words shifted.