I think he said they can still crow and make other vocalizations, but at reduced volume. However, I totally agree with your point that it still might be extremely frustrating for the bird, and I don't buy the argument that you can totally rule out frustration (or pain just after surgery) by observing the bird's behavior. To me it makes little sense to assume chickens always show how they feel. It's adaptive for people to show when they are suffering, because other people help them. But it's not adaptive for a chicken to show it, because their flock mates often bully or kill them, and predators might be attracted to them.
I would think of decrowing as a last resort procedure, and would favor not an out and out ban, but a requirement to show that it is a last resort. I believe this is how it works for debarking in Australia (?? I read about it several years ago, and not sure if my memory serves me correctly). I do think it's best to allow it as a last resort, because most animals would probably choose decrowing or debarking over death.