They can’t. Of course, they can’t really do it on a remote server either but they choose to pretend they can. If they really wanted to, they could run it through a machine learning system on your phone and just send the result to discord. That wouldn’t be secure, strictly speaking, but it would be good enough for this purpose. The kind of model they could send to a phone wouldn’t be very reliable, but that’s the problem they have anyway so nothing of value is lost