no I'm still here and it's still pretty clear to me exactly where it stands on that.
The aliens aren't any specific group of foreigners, they're the bad traits of every foreigner at once. They're the permanent other. Their defining cultural trait is "not Japanese." They're not specifically Kurds or Americans, they're Kurds AND Americans AND Koreans AND every other group Japanese people dislike dealing with.
They aren't even to be any sort of humans. As you said, they are just an absolute Other.
They are to be self-centered invaders trampling (unintentionally or not) on the locals.
The author is Japanese and primarily writing for a Japanese audience, so the behavior of the Other is based on things that the Japanese dislike about the non-Japanese visiting Japan - but not as a stand-in for them.
If I make a fantasy story with Orcs who are mounted marauding nomads, I might heavily use stuff from Ghengis-era mongolian tribes to flesh them out.
My Orcs aren't supposed to be Mongolians, or stand-ins for Mongolians, or to provide commentary on Mongolians, they are to be dangerous nomads and I just borrowed concepts from a previous group of dangerous nomads. I would likely also include aspects of Huns, Germanic/Gaul tribes, and Native American "Horse Tribes". But not as "these Orcs are literally X group" just "Oh this a thing a group did and it would make sense for a nomadic society based around war to also do".
So this isn't about Foreigners in Japan, this is about a guy reacting unhealthily to a changing world, using Aliens as a plot device and those aliens behaviors being what grinds the gears of the author and the people they are close with. And also maybe an Alien conspiracy; I'm pretty sure we're headed that way.