I have zero connection to the deal so this is entirely speculation, but in short, no, "acquired" has not become a dirty word. When a company is purchased - it's usually described as such. (In fact, I think for a public company, it's actually legally required that it be disclosed to shareholders in accurate terms, though the exact level of detail -- i.e. $$ -- is dependent on the "materiality" of the transaction.)

I noticed the exact same thing when I read the announcement(s) -- both Firebase and Google were very specific in saying that "Firebase has joined Google", not "Firebase has been acquired by Google".

So I'm guessing it means exactly what it says -- the Firebase team has been hired by Google and the product is now a part of Google Cloud Platform.