Let's take this apart slightly, since there's been a bit of confusion.
The word capable in the English language pretty much means you have the ability to. In this context, it is referencing the ability to design. Everyone is capable of making a design, yet, some designs are better than others based on your general knowledge. That is where an opinion is formed.
This is correct, they are entitled to an opinion, as not everyone has to be a winner. However, he does not have to shut up because he has an opinion about their opinion, as he is entitled to critique your opinion just as you are able to critique the original design (or anyone for that matter) in question.
In informal terms, this is known as arguing on the internet. It is my opinion that you have lost the original argument in question, what you choose to do as a result of my opinion (including making another opinion) is on the table.