I agree with you.
In the introduction page of XEBEC, it is clearly introduced as "Extraterrestrial_Intelligent_Life" (地球外の知的生命体).
Yes, that is the very definition of an Alien.
Also, in the #3 episode of the anime,
the interaction between Subaru and Omega-Xis(Warrock) in the house:
Omega-Xis : I didn't want to cause a scene when I came in contact with any Earthlings(地球人) other than you.
Subaru : Yeah.... I know , It's not like you're a bad Alien(宇宙人) or anything.
Then when Subaru used the Visibleizer:
Subaru : You are an Alien from the Denpa[Radio=EM]_World , huh?
Subaru : My dad used to experiment with Alien contact.
For reference, here's a line from Omega-Xis on the train:
Omega-Xis : I'm sure there are other FM coming to Earth, besides me. To destroy the Earth.
Omega-Xis : FM is belligerent, and I've heard that he wiped out a powerful planet like AM from the universe before I was born.
Omega-Xis : The radio virus was created by FM. As an indiscriminate attack to destroy other life forms.
In addition, in the game
https://youtu.be/r4XZbx1mDtc?t=366 ( Sorry, he's so impatient, I'll transcribe it for you... )
S:W..Who are you? キ、キミは だれ・・・？
o:My name is Warrock. You can call me "rock". オレの名はウォーロック、「ロック」と よんでくれ
o:I'm from FM Planet, the one you Earthlings call "the Alien". ＦＭプラネットから やってきた、オマエたちで いうところの うちゅう人ってヤツだ
o:Well, if you ask me, you[=Earthlings] are Alien as well. オレからいわせれば、オマエらも うちゅう人だがな
o:Before you ask, it's important to know that my body is made of radio waves..... いちおう、いっておくと、オレのカラダは電波でできていて.........
[ Note : うちゅう人=宇宙人 ]
No doubt, They used the word "Alien(宇宙人)".