@zyphil @ValiantHex yes, isekai means other world.
But "other world" does not refer to "another planet". The words "sekai" and "world" both refer to a certain time frame.
Basically, what the word
"world" translates as is "human age";
the "world" is the time since humans came into existence till the time humans cease to exist.
So isekai translates completely as "another human age", meaning that without humans it is not isekai.
However, I wouldn't deem this story really isekai, as it seems to exist in the same universe, and the ancient ruins seem to stem from the time when humanity spread through the universe to colonize these other planets, meaning it's more similar to Ginga Eiyuu Densetsu where different planets are inhabited by humans and one faction/organization tries to subjugate the colonies.
To really have an "isekai", it would need to be in another dimension separated from the other, and the transition is a one-time irreversible occurrence to start the story.