Spanish isn't the jumbled mess that English is. It has a lot of rhyme and reason to it. While not without exceptions, its rules are generally quite reliable.
"inx" is a not a Spanish syllable. You can't pluralize it. You can't rhyme with it. Hell, you can't barely pronounce it.
I get so tired of people popping up to defend the idea as if it's not so bad, invariably citing data collected from bilingual, second-generation US immigrants. Stop. If it's important to you, then at least have the sense to argue for "Latines", which at least kind of makes sense and doesn't sound like something invented by an English speaker who had only ever seen Spanish written, not spoken.
Your argument isn't cogent to me: it's an English word used in an English context. Your line of reasoning could be used to argue that "Anglo-Saxon"[1] is not a legitimate term because the Germanic tribes didn't use the term to refer to themselves.
1. The same goes for "German", "Chinese" or "Belter"
Latino and Latina are Spanish words that American English speakers have adopted in the last ~80 years due to the significant intermingling of Spanish and English speakers. Their use spans a spectrum of Spanish-only speakers, bilingual speakers, and English-only speakers. Not only does the English word "Latino" (my eyes roll typing that) have an etymological relationship with the Spanish word "Latino", their etymologies haven't even diverged. English speakers who took a Spanish class in high school even pronounce Latino in the Spanish way, "lah-TEE-noh" instead of "luh-TEE-no", because they think of it as a Spanish word.
I have friends who self-identify as Latinx, and use the word, I have friends who say Latine, I have friends who say Latino. Language evolves and I don't think your vitriol is warranted.
https://www.politico.com/news/2021/12/06/hispanic-voters-lat...