Indeed, try switching your smartphone to airplane mode and see how far your voice commands get you.

Did that (or rather disabled mobile data and WiFi, because airplane mode would still keep the WiFi on), and then I dictated this sentence after the parentheses. So Google’s voice input works offline just fine.

Or do they mean something like a smart assistant? In that case fair, but it’s not like it will work with text input either.

It is true, however, that Google Translate doesn’t do offline voice translation even if the language you’re trying to translate from is downloaded for system-wide voice recognition.