Earlier quoted context omitted.
Good catch! I didn't know about this, I'll update the footnote. Anecdotally everyone I know was very surprised by KEFIR the other day.
Because it's not a f*ckin' English word!
In english, "salsa" means a spicy dip made from peppers and other vegetables. In spanish, "salsa" just mean a sauce.
Loan words are a well estabished mechanism by which words enter a language from another langues. https://en.m.wikipedia.org/wiki/Loanword
Loanwords spread from their point of adoption into the wider language and can shift from the original meaning, just like other words.
"Kefir" has definitions in every major dictionary and is often used in contexts where all the other words are also english. That makes it pretty conclusively an English word.