You don't understand the question, hilariousandco is saying that you can view the list of sent messages even if you are banned. He is not talking about sending a message to anyone on the forum. Obviously, you are unlikely to be able to do that.
It does appear that he can't via the screenshot provided, but not allowing people to see their outbox seems a bit weird and it's the first I've heard of it. Don't suppose it matters either as the messages were reported.
I think he actually got blocked from the entire "My Messages"page, because normally, the forum's "address bar" will shows something like below, with its subsection being shown. I don't think we can simply choose "Personal Messages" section, it always followed by a subsection
But his shows like this,
