If the aliens aren't meant to be commentary on foreigners and instead an abstract "absolute other," then why are they an amalgam of the negative traits of foreigners with concrete references to specific foreigners?
For the same reason the orcs in my example might bear a passing resemblance to the Golden Horde despite not being intended to be mongolians.
In this case the author took the starting premise of Earth being first-contacted by aliens. "What if Aliens saved earth but rather than being absolutely kind altruists intent on bettering humanity, or nefarious invaders intent on enslaving/destorying humans, they were just really, really annoying and obnoxious?"
When creating the annoying "other" The japanese author is using what annoys Japanese about non-Japanese people and using that to build an archetype. But they aren't to be a stand in for the groups whose traits he's borrowing.
As I said before, if he was from colorado they would act like Californians but they aren't supposed to BE Californians.
They also act in ways that are NOT like any foreigner, and some of the behaviour is supposed to be just completely bizarre.
I mean by the basis of the aliens acting like IRL annoying tourist but the Japanese public still being polite and kind there is clearly commentary there about the Japanese being too soft on troublesome tourists. I wouldn't say the aliens are fully just meant to be foreigners or immigrants but they have those traits as well as other non-foreigner outsider traits like anti-social Japanese communities.
Saying that they don't hint to real life nucances in Japan is wrong just like saying the aliens only represent foreigners they're clearly meant to stand in for all kinds of problem people in Japanese society which includes toxic foreigners.
See my Orc example. They have selected traits from some groups because that is what the Author wants to convey, but they are not supposed to be/represent those groups.
If I made a character who was supposed to be "the annoying overly strict teacher" I would draw from from my least favorite educators for inspiration, but that character isn't supposed to be any of those people as I'd toss anyway any positive traits.
Its possible/likely there is some social commentary, but as is the case for a lot of Manga/Anime, Japan isn't Japan - Japan IS a stand in for the entirety of humanity.