I'm not sure I understand your point. Chinese is also not English but machine translation of Chinese to English can be done.
You're right that laypeople often assume, wrongly, that a given country's sign language is an encoding of the local spoken language. In reality it's usually a totally different language in its own right. But that shouldn't mean that translation is fundamentally impossible.
They didn't say it's fundamentally impossible, they said you need way more than 5000 hours to do it and that you'd need better quality sources than news interpreters.
I'm not sure I understand your point. Chinese is also not English but machine translation of Chinese to English can be done.
You're right that laypeople often assume, wrongly, that a given country's sign language is an encoding of the local spoken language. In reality it's usually a totally different language in its own right. But that shouldn't mean that translation is fundamentally impossible.