Is it true that there has never been a single case
It is tough to prove a negative. I am not going to completely parse the quote but please notice that the quote states "we couldn't find" and concludes that "it doesn't happen." Given these two pieces of information I do not conclude that there has never been a single case. Rather I conclude that the speaker in your quote could not find a case therefore he concluded that there has never been a single case.
It's largely impossible to determine that there has never been a single such case. We can search published opinions but that barely scratches the surface of lawsuits that are filed. It is entirely possible that someone filed a suit which was quickly dismissed.
The Act provides a defense, it does not bar lawsuits. Someone might get sick from food and not know where the food came from so they sue the provider. If this happens the provider may raise the Emerson Act as a defense and escape liability to the extent applicable. But again, we will never know because it's impossible to examine every lawsuit filed in this country.