NewUserNameHere Yes you're being unreasonable. The only purpose of language is to convey meaning. Beyond that, it makes no difference.
There's an entire area of linguistics called pragmatics that deals with the messages conveyed "between the lines", as it were - and that includes unintentional meanings. Like it or not, language does far more than convey meanings - it conveys ideas of competence, age, experience, reliability, and far more besides.
You understood the meaning perfectly. I'll never forget during my PhD (not linguistics) being told that "Linguists understand that languages change but it doesn't matter and is, in fact, a wonderful phenomenon. Let the rest of the world fret about it."
PhD in Linguistics. We also talk about linguistic norms and appropriacies for given contexts and purposes. You would (quite reasonably) be staggered if you received an email diagnosing you with a life-ending illness. You would also be horrified if your wedding invites cam back from the printers littered with typos. You probably wouldn't use really taboo language in front of children. It's not unreasonable for OP to expect the standard of English in a text from a medical business that she pays to reach some basic level of formality and standardness and to judge it accordingly if it falls short.
Purely guesswork as to if it's applicable, but another mis-remembered quote about not mocking anyone with pigeon English
Based on a single text that's a pretty brave conclusion. I'd like to see your evidence that this is pidgin English. I don't see any L2 in there.
because "they've already mastered one language" seems fairly apt here and the message you received seems to have many markers of the sender being a native speaker of a romantic language and you're being quite sneering OP.
Also interesting. Native-language identification of an L1 from an L2 is an interesting area in linguistics. What tells you that this person speaks a Romance language, and wasn't just rushing? It could have started as "I am texting to inform you" (ergo the grammatical error tells us nothing there) and "sorry for inconvenience" also tells us nothing because it's common to drop determiners in shortened messages. What are these many markers you're seeing?
I see at least two possibilities. (1) The person sending the text is a native speaker of English, rushed this out, and/or doesn't care about the presentation anyway. (2) The person sending the text is a non-native speaker of English with a wide range of possible L1s, and should have had their message checked before sending, or shouldn't have been given this task in the first place.
In either case, OP still has the right to form her own judgements.