While possible, this is not theoretically necessary, and I find this highly doubtful in practice.
What about the delay on each voice command as it pings the server? If it was generally the case that voice commands were sent remotely, I suggest usability would be 0.
There are many services that provide real time processing of large amounts of data on server cluster/farms, and more are coming. It is very much possible to perform large computations in almost real time in these situations, and you'd be surprised at how little latency people notice, particularly when there's no indication of what the actual number is.
I'm surprised to learn that people do use Siri in genuine situations, so thanks for your reply in that regard; although I still feel gratified that another commenter has the same experience that I do.
Judge the delay for yourself.
Relying on the internet for "real time" computation is a recipe for inconsistent behavior.